mTHP allocation stats for file-backed memory

Let's move count_mthp_stat() so that it's always defined, even when THP is disabled. Previously uses of the function in files such as shmem.c, which are compiled even when THP is disabled, required ugly THP ifdeferry. With this cleanup, we can remove those ifdefs and the function resolves to a nop when THP is disabled.

I shortly plan to call count_mthp_stat() from more THP-invariant source files.

Signed-off-by Ryan Roberts <[email protected]>

---
 include/linux/huge_mm.h | 70 ++++++++++++++++++++---------------------
 mm/memory.c             |  2 --
 mm/shmem.c              |  6 ----
 3 files changed, 35 insertions(+), 43 deletions(-)

include/linux/huge_mm.h +35 -35

index cff002be83eb..cb93b9009ce4 100644
@@ -108,6 +108,41 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
 #define HPAGE_PUD_MASK (~(HPAGE_PUD_SIZE - 1))
 #define HPAGE_PUD_SIZE ((1UL) << HPAGE_PUD_SHIFT)
 
+enum mthp_stat_item {
+       MTHP_STAT_ANON_FAULT_ALLOC,
+       MTHP_STAT_ANON_FAULT_FALLBACK,
+       MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
+       MTHP_STAT_SWPOUT,
+       MTHP_STAT_SWPOUT_FALLBACK,
+       MTHP_STAT_SHMEM_ALLOC,
+       MTHP_STAT_SHMEM_FALLBACK,
+       MTHP_STAT_SHMEM_FALLBACK_CHARGE,
+       MTHP_STAT_SPLIT,
+       MTHP_STAT_SPLIT_FAILED,
+       MTHP_STAT_SPLIT_DEFERRED,
+       __MTHP_STAT_COUNT
+};
+
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_SYSFS)
+struct mthp_stat {
+       unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
+};
+
+DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
+
+static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+{
+       if (order <= 0 || order > PMD_ORDER)
+               return;
+
+       this_cpu_inc(mthp_stats.stats[order][item]);
+}
+#else
+static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+{
+}
+#endif
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 
 extern unsigned long transparent_hugepage_flags;

@@ -263,41 +298,6 @@ struct thpsize {
 
 #define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
 
-enum mthp_stat_item {
-       MTHP_STAT_ANON_FAULT_ALLOC,
-       MTHP_STAT_ANON_FAULT_FALLBACK,
-       MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
-       MTHP_STAT_SWPOUT,
-       MTHP_STAT_SWPOUT_FALLBACK,
-       MTHP_STAT_SHMEM_ALLOC,
-       MTHP_STAT_SHMEM_FALLBACK,
-       MTHP_STAT_SHMEM_FALLBACK_CHARGE,
-       MTHP_STAT_SPLIT,
-       MTHP_STAT_SPLIT_FAILED,
-       MTHP_STAT_SPLIT_DEFERRED,
-       __MTHP_STAT_COUNT
-};
-
-struct mthp_stat {
-       unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
-};
-
-#ifdef CONFIG_SYSFS
-DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
-
-static inline void count_mthp_stat(int order, enum mthp_stat_item item)
-{
-       if (order <= 0 || order > PMD_ORDER)
-               return;
-
-       this_cpu_inc(mthp_stats.stats[order][item]);
-}
-#else
-static inline void count_mthp_stat(int order, enum mthp_stat_item item)
-{
-}
-#endif
-
 #define transparent_hugepage_use_zero_page()                           \
        (transparent_hugepage_flags &                                   \
         (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG))

mm/memory.c +0 -2

index 802d0d8a40f9..a50fdefb8f0b 100644
@@ -4597,9 +4597,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 
        folio_ref_add(folio, nr_pages - 1);
        add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
        count_mthp_stat(folio_order(folio), MTHP_STAT_ANON_FAULT_ALLOC);
-#endif
        folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
        folio_add_lru_vma(folio, vma);
 setpte:

mm/shmem.c +0 -7

index f24dfbd387ba..fce1343f44e6 100644
@@ -1776,9 +1776,7 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
 
                        if (pages == HPAGE_PMD_NR)
                                count_vm_event(THP_FILE_FALLBACK);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
                        count_mthp_stat(order, MTHP_STAT_SHMEM_FALLBACK);
-#endif
                        order = next_order(&suitable_orders, order);
                }
        } else {

@@ -1803,10 +1801,8 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
                                count_vm_event(THP_FILE_FALLBACK);
                                count_vm_event(THP_FILE_FALLBACK_CHARGE);
                        }
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
                        count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK);
                        count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK_CHARGE);
-#endif
                }
                goto unlock;
        }

@@ -2180,9 +2176,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
                if (!IS_ERR(folio)) {
                        if (folio_test_pmd_mappable(folio))
                                count_vm_event(THP_FILE_ALLOC);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
                        count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_ALLOC);
-#endif
                        goto alloced;
                }
                if (PTR_ERR(folio) == -EEXIST)
--

2.43.0

Let's move count_mthp_stat() so that it's always defined, even when THP is disabled. Previously uses of the function in files such as shmem.c, which are compiled even when THP is disabled, required ugly THP ifdeferry. With this cleanup, we can remove those ifdefs and the function resolves to a nop when THP is disabled.

I shortly plan to call count_mthp_stat() from more THP-invariant source files.

Signed-off-by Ryan Roberts <[email protected]>

---
  include/linux/huge_mm.h | 70 ++++++++++++++++++++---------------------
  mm/memory.c             |  2 --
  mm/shmem.c              |  6 ----
  3 files changed, 35 insertions(+), 43 deletions(-)

include/linux/huge_mm.h +35 -35

index cff002be83eb..cb93b9009ce4 100644
@@ -108,6 +108,41 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
  #define HPAGE_PUD_MASK	(~(HPAGE_PUD_SIZE - 1))
  #define HPAGE_PUD_SIZE	((1UL) << HPAGE_PUD_SHIFT)
 
+enum mthp_stat_item {
+	MTHP_STAT_ANON_FAULT_ALLOC,
+	MTHP_STAT_ANON_FAULT_FALLBACK,
+	MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
+	MTHP_STAT_SWPOUT,
+	MTHP_STAT_SWPOUT_FALLBACK,
+	MTHP_STAT_SHMEM_ALLOC,
+	MTHP_STAT_SHMEM_FALLBACK,
+	MTHP_STAT_SHMEM_FALLBACK_CHARGE,
+	MTHP_STAT_SPLIT,
+	MTHP_STAT_SPLIT_FAILED,
+	MTHP_STAT_SPLIT_DEFERRED,
+	__MTHP_STAT_COUNT
+};
+
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_SYSFS)
+struct mthp_stat {
+	unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
+};
+
+DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
+
+static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+{
+	if (order <= 0 || order > PMD_ORDER)
+		return;
+
+	this_cpu_inc(mthp_stats.stats[order][item]);
+}
+#else
+static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+{
+}
+#endif
+
  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 
  extern unsigned long transparent_hugepage_flags;

@@ -263,41 +298,6 @@ struct thpsize {
 
  #define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
 
-enum mthp_stat_item {
-	MTHP_STAT_ANON_FAULT_ALLOC,
-	MTHP_STAT_ANON_FAULT_FALLBACK,
-	MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
-	MTHP_STAT_SWPOUT,
-	MTHP_STAT_SWPOUT_FALLBACK,
-	MTHP_STAT_SHMEM_ALLOC,
-	MTHP_STAT_SHMEM_FALLBACK,
-	MTHP_STAT_SHMEM_FALLBACK_CHARGE,
-	MTHP_STAT_SPLIT,
-	MTHP_STAT_SPLIT_FAILED,
-	MTHP_STAT_SPLIT_DEFERRED,
-	__MTHP_STAT_COUNT
-};
-
-struct mthp_stat {
-	unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
-};
-
-#ifdef CONFIG_SYSFS
-DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
-
-static inline void count_mthp_stat(int order, enum mthp_stat_item item)
-{
-	if (order <= 0 || order > PMD_ORDER)
-		return;
-
-	this_cpu_inc(mthp_stats.stats[order][item]);
-}
-#else
-static inline void count_mthp_stat(int order, enum mthp_stat_item item)
-{
-}
-#endif
-
  #define transparent_hugepage_use_zero_page()				\
  	(transparent_hugepage_flags &					\
  	 (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG))

mm/memory.c +0 -2

index 802d0d8a40f9..a50fdefb8f0b 100644
@@ -4597,9 +4597,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 
  	folio_ref_add(folio, nr_pages - 1);
  	add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
  	count_mthp_stat(folio_order(folio), MTHP_STAT_ANON_FAULT_ALLOC);
-#endif
  	folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
  	folio_add_lru_vma(folio, vma);
  setpte:

mm/shmem.c +0 -6

index f24dfbd387ba..fce1343f44e6 100644
@@ -1776,9 +1776,7 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
 
  			if (pages == HPAGE_PMD_NR)
  				count_vm_event(THP_FILE_FALLBACK);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
  			count_mthp_stat(order, MTHP_STAT_SHMEM_FALLBACK);
-#endif
  			order = next_order(&suitable_orders, order);
  		}
  	} else {

@@ -1803,10 +1801,8 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
  				count_vm_event(THP_FILE_FALLBACK);
  				count_vm_event(THP_FILE_FALLBACK_CHARGE);
  			}
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
  			count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK);
  			count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK_CHARGE);
-#endif
  		}
  		goto unlock;
  	}

@@ -2180,9 +2176,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
  		if (!IS_ERR(folio)) {
  			if (folio_test_pmd_mappable(folio))
  				count_vm_event(THP_FILE_ALLOC);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
  			count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_ALLOC);
-#endif
  			goto alloced;
  		}
  		if (PTR_ERR(folio) == -EEXIST)

Let's move count_mthp_stat() so that it's always defined, even when THP is disabled. Previously uses of the function in files such as shmem.c, which are compiled even when THP is disabled, required ugly THP ifdeferry. With this cleanup, we can remove those ifdefs and the function resolves to a nop when THP is disabled.

I shortly plan to call count_mthp_stat() from more THP-invariant source files.

Signed-off-by Ryan Roberts <[email protected]>

---
 include/linux/huge_mm.h | 70 ++++++++++++++++++++---------------------
 mm/memory.c             |  2 --
 mm/shmem.c              |  6 ----
 3 files changed, 35 insertions(+), 43 deletions(-)

include/linux/huge_mm.h +35 -35

index cff002be83eb..cb93b9009ce4 100644
@@ -108,6 +108,41 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
 #define HPAGE_PUD_MASK (~(HPAGE_PUD_SIZE - 1))
 #define HPAGE_PUD_SIZE ((1UL) << HPAGE_PUD_SHIFT)
 
+enum mthp_stat_item {
+       MTHP_STAT_ANON_FAULT_ALLOC,
+       MTHP_STAT_ANON_FAULT_FALLBACK,
+       MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
+       MTHP_STAT_SWPOUT,
+       MTHP_STAT_SWPOUT_FALLBACK,
+       MTHP_STAT_SHMEM_ALLOC,
+       MTHP_STAT_SHMEM_FALLBACK,
+       MTHP_STAT_SHMEM_FALLBACK_CHARGE,
+       MTHP_STAT_SPLIT,
+       MTHP_STAT_SPLIT_FAILED,
+       MTHP_STAT_SPLIT_DEFERRED,
+       __MTHP_STAT_COUNT
+};
+
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_SYSFS)
+struct mthp_stat {
+       unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
+};
+
+DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
+
+static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+{
+       if (order <= 0 || order > PMD_ORDER)
+               return;
+
+       this_cpu_inc(mthp_stats.stats[order][item]);
+}
+#else
+static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+{
+}
+#endif
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 
 extern unsigned long transparent_hugepage_flags;

@@ -263,41 +298,6 @@ struct thpsize {
 
 #define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
 
-enum mthp_stat_item {
-       MTHP_STAT_ANON_FAULT_ALLOC,
-       MTHP_STAT_ANON_FAULT_FALLBACK,
-       MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
-       MTHP_STAT_SWPOUT,
-       MTHP_STAT_SWPOUT_FALLBACK,
-       MTHP_STAT_SHMEM_ALLOC,
-       MTHP_STAT_SHMEM_FALLBACK,
-       MTHP_STAT_SHMEM_FALLBACK_CHARGE,
-       MTHP_STAT_SPLIT,
-       MTHP_STAT_SPLIT_FAILED,
-       MTHP_STAT_SPLIT_DEFERRED,
-       __MTHP_STAT_COUNT
-};
-
-struct mthp_stat {
-       unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
-};
-
-#ifdef CONFIG_SYSFS
-DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
-
-static inline void count_mthp_stat(int order, enum mthp_stat_item item)
-{
-       if (order <= 0 || order > PMD_ORDER)
-               return;
-
-       this_cpu_inc(mthp_stats.stats[order][item]);
-}
-#else
-static inline void count_mthp_stat(int order, enum mthp_stat_item item)
-{
-}
-#endif
-
 #define transparent_hugepage_use_zero_page()                           \
        (transparent_hugepage_flags &                                   \
         (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG))

mm/memory.c +0 -2

index 802d0d8a40f9..a50fdefb8f0b 100644
@@ -4597,9 +4597,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 
        folio_ref_add(folio, nr_pages - 1);
        add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
        count_mthp_stat(folio_order(folio), MTHP_STAT_ANON_FAULT_ALLOC);
-#endif
        folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
        folio_add_lru_vma(folio, vma);
 setpte:

mm/shmem.c +0 -7

index f24dfbd387ba..fce1343f44e6 100644
@@ -1776,9 +1776,7 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
 
                        if (pages == HPAGE_PMD_NR)
                                count_vm_event(THP_FILE_FALLBACK);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
                        count_mthp_stat(order, MTHP_STAT_SHMEM_FALLBACK);
-#endif
                        order = next_order(&suitable_orders, order);
                }
        } else {

@@ -1803,10 +1801,8 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
                                count_vm_event(THP_FILE_FALLBACK);
                                count_vm_event(THP_FILE_FALLBACK_CHARGE);
                        }
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
                        count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK);
                        count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK_CHARGE);
-#endif
                }
                goto unlock;
        }

@@ -2180,9 +2176,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
                if (!IS_ERR(folio)) {
                        if (folio_test_pmd_mappable(folio))
                                count_vm_event(THP_FILE_ALLOC);
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
                        count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_ALLOC);
-#endif
                        goto alloced;
                }
                if (PTR_ERR(folio) == -EEXIST)
--

2.43.0

Let's move count_mthp_stat() so that it's always defined, even when THP is disabled. Previously uses of the function in files such as shmem.c, which are compiled even when THP is disabled, required ugly THP ifdeferry. With this cleanup, we can remove those ifdefs and the function resolves to a nop when THP is disabled.

I shortly plan to call count_mthp_stat() from more THP-invariant source files.

Signed-off-by Ryan Roberts <[email protected]>

---

Expose 3 new mTHP stats for file (pagecache) folio allocations:

/sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_alloc /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback_charge

This will provide some insight on the sizes of large folios being allocated for file-backed memory, and how often allocation is failing.

All non-order-0 (and most order-0) folio allocations are currently done through filemap_alloc_folio(), and folios are charged in a subsequent call to filemap_add_folio(). So count file_fallback when allocation fails in filemap_alloc_folio() and count file_alloc or file_fallback_charge in filemap_add_folio(), based on whether charging succeeded or not. There are some users of filemap_add_folio() that allocate their own order-0 folio by other means, so we would not count an allocation failure in this case, but we also don't care about order-0 allocations. This approach feels like it should be good enough and doesn't require any (impractically large) refactoring.

The existing mTHP stats interface is reused to provide consistency to users. And because we are reusing the same interface, we can reuse the same infrastructure on the kernel side. The one small wrinkle is that the set of folio sizes supported by the pagecache are not identical to those supported by anon and shmem; pagecache supports order-1, unlike anon and shmem, and the max pagecache order may be less than PMD-size (see arm64 with 64K base pages), again unlike anon and shmem. So we now create a hugepages-*kB directory for the union of the sizes supported by all 3 memory types and populate it with the relevant stats and controls.

Signed-off-by Ryan Roberts <[email protected]>

---
  Documentation/admin-guide/mm/transhuge.rst |  13 +++
  include/linux/huge_mm.h                    |   6 +-
  include/linux/pagemap.h                    |  17 ++-
  mm/filemap.c                               |   6 +-
  mm/huge_memory.c                           | 117 ++++++++++++++++-----
  5 files changed, 128 insertions(+), 31 deletions(-)

Documentation/admin-guide/mm/transhuge.rst +13 -0

index 058485daf186..d4857e457add 100644
@@ -512,6 +512,19 @@ shmem_fallback_charge
  	falls back to using small pages even though the allocation was
  	successful.
 
+file_alloc
+	is incremented every time a file huge page is successfully
+	allocated.
+
+file_fallback
+	is incremented if a file huge page is attempted to be allocated
+	but fails and instead falls back to using small pages.
+
+file_fallback_charge
+	is incremented if a file huge page cannot be charged and instead
+	falls back to using small pages even though the allocation was
+	successful.
+
  split
  	is incremented every time a huge page is successfully split into
  	smaller orders. This can happen for a variety of reasons but a

include/linux/huge_mm.h +4 -2

index cb93b9009ce4..b4fba11976f2 100644
@@ -117,6 +117,9 @@ enum mthp_stat_item {
  	MTHP_STAT_SHMEM_ALLOC,
  	MTHP_STAT_SHMEM_FALLBACK,
  	MTHP_STAT_SHMEM_FALLBACK_CHARGE,
+	MTHP_STAT_FILE_ALLOC,
+	MTHP_STAT_FILE_FALLBACK,
+	MTHP_STAT_FILE_FALLBACK_CHARGE,
  	MTHP_STAT_SPLIT,
  	MTHP_STAT_SPLIT_FAILED,
  	MTHP_STAT_SPLIT_DEFERRED,

@@ -292,11 +295,10 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
 
  struct thpsize {
  	struct kobject kobj;
-	struct list_head node;
  	int order;
  };
 
-#define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
+#define to_thpsize(_kobj) container_of(_kobj, struct thpsize, kobj)
 
  #define transparent_hugepage_use_zero_page()				\
  	(transparent_hugepage_flags &					\

include/linux/pagemap.h +15 -2

index 6e2f72d03176..f45a1ba6d9b6 100644
@@ -365,6 +365,7 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
   */
  #define MAX_XAS_ORDER		(XA_CHUNK_SHIFT * 2 - 1)
  #define MAX_PAGECACHE_ORDER	min(MAX_XAS_ORDER, PREFERRED_MAX_PAGECACHE_ORDER)
+#define PAGECACHE_LARGE_ORDERS	((BIT(MAX_PAGECACHE_ORDER + 1) - 1) & ~BIT(0))
 
  /**
   * mapping_set_large_folios() - Indicate the file supports large folios.

@@ -562,14 +563,26 @@ static inline void *detach_page_private(struct page *page)
  }
 
  #ifdef CONFIG_NUMA
-struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order);
+struct folio *__filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order);
  #else
-static inline struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
+static inline struct folio *__filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
  {
  	return folio_alloc_noprof(gfp, order);
  }
  #endif
 
+static inline struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
+{
+	struct folio *folio;
+
+	folio = __filemap_alloc_folio_noprof(gfp, order);
+
+	if (!folio)
+		count_mthp_stat(order, MTHP_STAT_FILE_FALLBACK);
+
+	return folio;
+}
+
  #define filemap_alloc_folio(...)				\
  	alloc_hooks(filemap_alloc_folio_noprof(__VA_ARGS__))
 

mm/filemap.c +4 -2

index 53d5d0410b51..131d514fca29 100644
@@ -963,6 +963,8 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
  	int ret;
 
  	ret = mem_cgroup_charge(folio, NULL, gfp);
+	count_mthp_stat(folio_order(folio),
+		ret ? MTHP_STAT_FILE_FALLBACK_CHARGE : MTHP_STAT_FILE_ALLOC);
  	if (ret)
  		return ret;
 

@@ -990,7 +992,7 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
  EXPORT_SYMBOL_GPL(filemap_add_folio);
 
  #ifdef CONFIG_NUMA
-struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
+struct folio *__filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
  {
  	int n;
  	struct folio *folio;

@@ -1007,7 +1009,7 @@ struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
  	}
  	return folio_alloc_noprof(gfp, order);
  }
-EXPORT_SYMBOL(filemap_alloc_folio_noprof);
+EXPORT_SYMBOL(__filemap_alloc_folio_noprof);
  #endif
 
  /*

mm/huge_memory.c +67 -15

index f9696c94e211..559553e2a662 100644
@@ -452,8 +452,9 @@ static const struct attribute_group hugepage_attr_group = {
 
  static void hugepage_exit_sysfs(struct kobject *hugepage_kobj);
  static void thpsize_release(struct kobject *kobj);
+static void thpsize_child_release(struct kobject *kobj);
  static DEFINE_SPINLOCK(huge_anon_orders_lock);
-static LIST_HEAD(thpsize_list);
+static LIST_HEAD(thpsize_child_list);
 
  static ssize_t thpsize_enabled_show(struct kobject *kobj,
  				    struct kobj_attribute *attr, char *buf)

@@ -537,6 +538,18 @@ static const struct kobj_type thpsize_ktype = {
  	.sysfs_ops = &kobj_sysfs_ops,
  };
 
+static const struct kobj_type thpsize_child_ktype = {
+	.release = &thpsize_child_release,
+	.sysfs_ops = &kobj_sysfs_ops,
+};
+
+struct thpsize_child {
+	struct kobject kobj;
+	struct list_head node;
+};
+
+#define to_thpsize_child(_kobj) container_of(_kobj, struct thpsize, kobj)
+
  DEFINE_PER_CPU(struct mthp_stat, mthp_stats) = {{{0}}};
 
  static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)

@@ -557,7 +570,7 @@ static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)
  static ssize_t _name##_show(struct kobject *kobj,			\
  			struct kobj_attribute *attr, char *buf)		\
  {									\
-	int order = to_thpsize(kobj)->order;				\
+	int order = to_thpsize(kobj->parent)->order;			\
  									\
  	return sysfs_emit(buf, "%lu\n", sum_mthp_stat(order, _index));	\
  }									\

@@ -591,41 +604,93 @@ static struct attribute *stats_attrs[] = {
  };
 
  static struct attribute_group stats_attr_group = {
-	.name = "stats",
  	.attrs = stats_attrs,
  };
 
-static struct thpsize *thpsize_create(int order, struct kobject *parent)
+DEFINE_MTHP_STAT_ATTR(file_alloc, MTHP_STAT_FILE_ALLOC);
+DEFINE_MTHP_STAT_ATTR(file_fallback, MTHP_STAT_FILE_FALLBACK);
+DEFINE_MTHP_STAT_ATTR(file_fallback_charge, MTHP_STAT_FILE_FALLBACK_CHARGE);
+
+static struct attribute *file_stats_attrs[] = {
+	&file_alloc_attr.attr,
+	&file_fallback_attr.attr,
+	&file_fallback_charge_attr.attr,
+	NULL,
+};
+
+static struct attribute_group file_stats_attr_group = {
+	.attrs = file_stats_attrs,
+};
+
+static int thpsize_create(int order, struct kobject *parent)
  {
  	unsigned long size = (PAGE_SIZE << order) / SZ_1K;
+	struct thpsize_child *stats;
  	struct thpsize *thpsize;
  	int ret;
 
+	/*
+	 * Each child object (currently only "stats" directory) holds a
+	 * reference to the top-level thpsize object, so we can drop our ref to
+	 * the top-level once stats is setup. Then we just need to drop a
+	 * reference on any children to clean everything up. We can't just use
+	 * the attr group name for the stats subdirectory because there may be
+	 * multiple attribute groups to populate inside stats and overlaying
+	 * using the name property isn't supported in that way; each attr group
+	 * name, if provided, must be unique in the parent directory.
+	 */
+
  	thpsize = kzalloc(sizeof(*thpsize), GFP_KERNEL);
-	if (!thpsize)
-		return ERR_PTR(-ENOMEM);
+	if (!thpsize) {
+		ret = -ENOMEM;
+		goto err;
+	}
+	thpsize->order = order;
 
  	ret = kobject_init_and_add(&thpsize->kobj, &thpsize_ktype, parent,
  				   "hugepages-%lukB", size);
  	if (ret) {
  		kfree(thpsize);
-		return ERR_PTR(ret);
+		goto err;
  	}
 
-	ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
-	if (ret) {
+	stats = kzalloc(sizeof(*stats), GFP_KERNEL);
+	if (!stats) {
  		kobject_put(&thpsize->kobj);
-		return ERR_PTR(ret);
+		ret = -ENOMEM;
+		goto err;
  	}
 
-	ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group);
+	ret = kobject_init_and_add(&stats->kobj, &thpsize_child_ktype,
+				   &thpsize->kobj, "stats");
+	kobject_put(&thpsize->kobj);
  	if (ret) {
-		kobject_put(&thpsize->kobj);
-		return ERR_PTR(ret);
+		kfree(stats);
+		goto err;
  	}
 
-	thpsize->order = order;
-	return thpsize;
+	if (BIT(order) & THP_ORDERS_ALL_ANON) {
+		ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
+		if (ret)
+			goto err_put;
+
+		ret = sysfs_create_group(&stats->kobj, &stats_attr_group);
+		if (ret)
+			goto err_put;
+	}

mm/huge_memory.c +15 -10

+	kobject_put(&stats->kobj);
+err:
+	return ret;
  }
 
  static void thpsize_release(struct kobject *kobj)

@@ -633,10 +698,14 @@ static void thpsize_release(struct kobject *kobj)
  	kfree(to_thpsize(kobj));
  }
 
+static void thpsize_child_release(struct kobject *kobj)
+{
+	kfree(to_thpsize_child(kobj));
+}
+
  static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
  {
  	int err;
-	struct thpsize *thpsize;
  	unsigned long orders;
  	int order;
 

@@ -665,16 +734,14 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
  		goto remove_hp_group;
  	}
 
-	orders = THP_ORDERS_ALL_ANON;
+	orders = THP_ORDERS_ALL_ANON | PAGECACHE_LARGE_ORDERS;
  	order = highest_order(orders);
  	while (orders) {
-		thpsize = thpsize_create(order, *hugepage_kobj);
-		if (IS_ERR(thpsize)) {
+		err = thpsize_create(order, *hugepage_kobj);
+		if (err) {
  			pr_err("failed to create thpsize for order %d\n", order);
-			err = PTR_ERR(thpsize);
  			goto remove_all;
  		}
-		list_add(&thpsize->node, &thpsize_list);
  		order = next_order(&orders, order);
  	}
 

@@ -692,11 +759,11 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
 
  static void __init hugepage_exit_sysfs(struct kobject *hugepage_kobj)
  {
-	struct thpsize *thpsize, *tmp;
+	struct thpsize_child *child, *tmp;
 
-	list_for_each_entry_safe(thpsize, tmp, &thpsize_list, node) {
-		list_del(&thpsize->node);
-		kobject_put(&thpsize->kobj);
+	list_for_each_entry_safe(child, tmp, &thpsize_child_list, node) {
+		list_del(&child->node);
+		kobject_put(&child->kobj);
  	}
 
  	sysfs_remove_group(hugepage_kobj, &khugepaged_attr_group);

Quoted 20 lines · Ryan Roberts

Expose 3 new mTHP stats for file (pagecache) folio allocations:

/sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_alloc /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback_charge

This will provide some insight on the sizes of large folios being allocated for file-backed memory, and how often allocation is failing.

All non-order-0 (and most order-0) folio allocations are currently done through filemap_alloc_folio(), and folios are charged in a subsequent call to filemap_add_folio(). So count file_fallback when allocation fails in filemap_alloc_folio() and count file_alloc or file_fallback_charge in filemap_add_folio(), based on whether charging succeeded or not. There are some users of filemap_add_folio() that allocate their own order-0 folio by other means, so we would not count an allocation failure in this case, but we also don't care about order-0 allocations. This approach feels like it should be good enough and doesn't require any (impractically large) refactoring.

Quoted 9 lines

The existing mTHP stats interface is reused to provide consistency to users. And because we are reusing the same interface, we can reuse the same infrastructure on the kernel side. The one small wrinkle is that the set of folio sizes supported by the pagecache are not identical to those supported by anon and shmem; pagecache supports order-1, unlike anon and shmem, and the max pagecache order may be less than PMD-size (see arm64 with 64K base pages), again unlike anon and shmem. So we now create a hugepages-*kB directory for the union of the sizes supported by all 3 memory types and populate it with the relevant stats and controls.

Personally, I like the idea that can help analyze the allocation of large folios for the page cache.

However, I have a slight concern about the consistency of the interface.

For 64K, the fields layout:

├── hugepages-64kB
│   ├── enabled
│   ├── shmem_enabled
│   └── stats
│       ├── anon_fault_alloc
│       ├── anon_fault_fallback
│       ├── anon_fault_fallback_charge
│       ├── file_alloc
│       ├── file_fallback
│       ├── file_fallback_charge
│       ├── shmem_alloc
│       ├── shmem_fallback
│       ├── shmem_fallback_charge
│       ├── split
│       ├── split_deferred
│       ├── split_failed
│       ├── swpout
│       └── swpout_fallback

But for 8K (for pagecache), you removed some fields (of course, I understand why they are not supported).

├── hugepages-8kB
│   └── stats
│       ├── file_alloc
│       ├── file_fallback
│       └── file_fallback_charge

This might not be user-friendly for some user-space parsing tools, as they lack certain fields for the same pattern interfaces. Of course, this might not be an issue if we have clear documentation describing the differences here:)

Another possible approach is to maintain the same field layout to keep consistent, but prohibit writing to the fields that are not supported by the pagecache, and any stats read from them would be 0.

Quoted 268 lines

Signed-off-by Ryan Roberts <[email protected]>

---
  Documentation/admin-guide/mm/transhuge.rst |  13 +++
  include/linux/huge_mm.h                    |   6 +-
  include/linux/pagemap.h                    |  17 ++-
  mm/filemap.c                               |   6 +-
  mm/huge_memory.c                           | 117 ++++++++++++++++-----
  5 files changed, 128 insertions(+), 31 deletions(-)

Documentation/admin-guide/mm/transhuge.rst +13 -0

index 058485daf186..d4857e457add 100644
@@ -512,6 +512,19 @@ shmem_fallback_charge
      falls back to using small pages even though the allocation was
      successful.
 
+file_alloc
+     is incremented every time a file huge page is successfully
+     allocated.
+
+file_fallback
+     is incremented if a file huge page is attempted to be allocated
+     but fails and instead falls back to using small pages.
+
+file_fallback_charge
+     is incremented if a file huge page cannot be charged and instead
+     falls back to using small pages even though the allocation was
+     successful.
+
  split
      is incremented every time a huge page is successfully split into
      smaller orders. This can happen for a variety of reasons but a

include/linux/huge_mm.h +4 -2

index cb93b9009ce4..b4fba11976f2 100644
@@ -117,6 +117,9 @@ enum mthp_stat_item {
      MTHP_STAT_SHMEM_ALLOC,
      MTHP_STAT_SHMEM_FALLBACK,
      MTHP_STAT_SHMEM_FALLBACK_CHARGE,
+     MTHP_STAT_FILE_ALLOC,
+     MTHP_STAT_FILE_FALLBACK,
+     MTHP_STAT_FILE_FALLBACK_CHARGE,
      MTHP_STAT_SPLIT,
      MTHP_STAT_SPLIT_FAILED,
      MTHP_STAT_SPLIT_DEFERRED,

@@ -292,11 +295,10 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
 
  struct thpsize {
      struct kobject kobj;
-     struct list_head node;
      int order;
  };
 
-#define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
+#define to_thpsize(_kobj) container_of(_kobj, struct thpsize, kobj)
 
  #define transparent_hugepage_use_zero_page()                                \
      (transparent_hugepage_flags &                                   \

include/linux/pagemap.h +15 -2

index 6e2f72d03176..f45a1ba6d9b6 100644
@@ -365,6 +365,7 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
   */
  #define MAX_XAS_ORDER               (XA_CHUNK_SHIFT * 2 - 1)
  #define MAX_PAGECACHE_ORDER min(MAX_XAS_ORDER, PREFERRED_MAX_PAGECACHE_ORDER)
+#define PAGECACHE_LARGE_ORDERS       ((BIT(MAX_PAGECACHE_ORDER + 1) - 1) & ~BIT(0))
 
  /**
   * mapping_set_large_folios() - Indicate the file supports large folios.

@@ -562,14 +563,26 @@ static inline void *detach_page_private(struct page *page)
  }
 
  #ifdef CONFIG_NUMA
-struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order);
+struct folio *__filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order);
  #else
-static inline struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
+static inline struct folio *__filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
  {
      return folio_alloc_noprof(gfp, order);
  }
  #endif
 
+static inline struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
+{
+     struct folio *folio;
+
+     folio = __filemap_alloc_folio_noprof(gfp, order);
+
+     if (!folio)
+             count_mthp_stat(order, MTHP_STAT_FILE_FALLBACK);
+
+     return folio;
+}
+
  #define filemap_alloc_folio(...)                            \
      alloc_hooks(filemap_alloc_folio_noprof(__VA_ARGS__))
 

mm/filemap.c +4 -2

index 53d5d0410b51..131d514fca29 100644
@@ -963,6 +963,8 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
      int ret;
 
      ret = mem_cgroup_charge(folio, NULL, gfp);
+     count_mthp_stat(folio_order(folio),
+             ret ? MTHP_STAT_FILE_FALLBACK_CHARGE : MTHP_STAT_FILE_ALLOC);
      if (ret)
              return ret;
 

@@ -990,7 +992,7 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
  EXPORT_SYMBOL_GPL(filemap_add_folio);
 
  #ifdef CONFIG_NUMA
-struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
+struct folio *__filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
  {
      int n;
      struct folio *folio;

@@ -1007,7 +1009,7 @@ struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
      }
      return folio_alloc_noprof(gfp, order);
  }
-EXPORT_SYMBOL(filemap_alloc_folio_noprof);
+EXPORT_SYMBOL(__filemap_alloc_folio_noprof);
  #endif
 
  /*

mm/huge_memory.c +67 -15

index f9696c94e211..559553e2a662 100644
@@ -452,8 +452,9 @@ static const struct attribute_group hugepage_attr_group = {
 
  static void hugepage_exit_sysfs(struct kobject *hugepage_kobj);
  static void thpsize_release(struct kobject *kobj);
+static void thpsize_child_release(struct kobject *kobj);
  static DEFINE_SPINLOCK(huge_anon_orders_lock);
-static LIST_HEAD(thpsize_list);
+static LIST_HEAD(thpsize_child_list);
 
  static ssize_t thpsize_enabled_show(struct kobject *kobj,
                                  struct kobj_attribute *attr, char *buf)

@@ -537,6 +538,18 @@ static const struct kobj_type thpsize_ktype = {
      .sysfs_ops = &kobj_sysfs_ops,
  };
 
+static const struct kobj_type thpsize_child_ktype = {
+     .release = &thpsize_child_release,
+     .sysfs_ops = &kobj_sysfs_ops,
+};
+
+struct thpsize_child {
+     struct kobject kobj;
+     struct list_head node;
+};
+
+#define to_thpsize_child(_kobj) container_of(_kobj, struct thpsize, kobj)
+
  DEFINE_PER_CPU(struct mthp_stat, mthp_stats) = {{{0}}};
 
  static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)

@@ -557,7 +570,7 @@ static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)
  static ssize_t _name##_show(struct kobject *kobj,                   \
                      struct kobj_attribute *attr, char *buf)         \
  {                                                                   \
-     int order = to_thpsize(kobj)->order;                            \
+     int order = to_thpsize(kobj->parent)->order;                    \
                                                                      \
      return sysfs_emit(buf, "%lu\n", sum_mthp_stat(order, _index));  \
  }                                                                   \

@@ -591,41 +604,93 @@ static struct attribute *stats_attrs[] = {
  };
 
  static struct attribute_group stats_attr_group = {
-     .name = "stats",
      .attrs = stats_attrs,
  };
 
-static struct thpsize *thpsize_create(int order, struct kobject *parent)
+DEFINE_MTHP_STAT_ATTR(file_alloc, MTHP_STAT_FILE_ALLOC);
+DEFINE_MTHP_STAT_ATTR(file_fallback, MTHP_STAT_FILE_FALLBACK);
+DEFINE_MTHP_STAT_ATTR(file_fallback_charge, MTHP_STAT_FILE_FALLBACK_CHARGE);
+
+static struct attribute *file_stats_attrs[] = {
+     &file_alloc_attr.attr,
+     &file_fallback_attr.attr,
+     &file_fallback_charge_attr.attr,
+     NULL,
+};
+
+static struct attribute_group file_stats_attr_group = {
+     .attrs = file_stats_attrs,
+};
+
+static int thpsize_create(int order, struct kobject *parent)
  {
      unsigned long size = (PAGE_SIZE << order) / SZ_1K;
+     struct thpsize_child *stats;
      struct thpsize *thpsize;
      int ret;
 
+     /*
+      * Each child object (currently only "stats" directory) holds a
+      * reference to the top-level thpsize object, so we can drop our ref to
+      * the top-level once stats is setup. Then we just need to drop a
+      * reference on any children to clean everything up. We can't just use
+      * the attr group name for the stats subdirectory because there may be
+      * multiple attribute groups to populate inside stats and overlaying
+      * using the name property isn't supported in that way; each attr group
+      * name, if provided, must be unique in the parent directory.
+      */
+
      thpsize = kzalloc(sizeof(*thpsize), GFP_KERNEL);
-     if (!thpsize)
-             return ERR_PTR(-ENOMEM);
+     if (!thpsize) {
+             ret = -ENOMEM;
+             goto err;
+     }
+     thpsize->order = order;
 
      ret = kobject_init_and_add(&thpsize->kobj, &thpsize_ktype, parent,
                                 "hugepages-%lukB", size);
      if (ret) {
              kfree(thpsize);
-             return ERR_PTR(ret);
+             goto err;
      }
 
-     ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
-     if (ret) {
+     stats = kzalloc(sizeof(*stats), GFP_KERNEL);
+     if (!stats) {
              kobject_put(&thpsize->kobj);
-             return ERR_PTR(ret);
+             ret = -ENOMEM;
+             goto err;
      }
 
-     ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group);
+     ret = kobject_init_and_add(&stats->kobj, &thpsize_child_ktype,
+                                &thpsize->kobj, "stats");
+     kobject_put(&thpsize->kobj);
      if (ret) {
-             kobject_put(&thpsize->kobj);
-             return ERR_PTR(ret);
+             kfree(stats);
+             goto err;
      }
 
-     thpsize->order = order;
-     return thpsize;
+     if (BIT(order) & THP_ORDERS_ALL_ANON) {
+             ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
+             if (ret)
+                     goto err_put;
+
+             ret = sysfs_create_group(&stats->kobj, &stats_attr_group);
+             if (ret)
+                     goto err_put;
+     }

mm/huge_memory.c static struct attribute *stats_attrs[] = { · line 690

Quoted 10 lines

mm/huge_memory.c +10 -0

+
+     if (BIT(order) & PAGECACHE_LARGE_ORDERS) {
+             ret = sysfs_create_group(&stats->kobj, &file_stats_attr_group);
+             if (ret)
+                     goto err_put;
+     }
+
+     list_add(&stats->node, &thpsize_child_list);
+     return 0;
+err_put:

IIUC, I think you should call 'sysfs_remove_group' to remove the group before putting the kobject.

Quoted 58 lines

mm/huge_memory.c +15 -10

+     kobject_put(&stats->kobj);
+err:
+     return ret;
  }
 
  static void thpsize_release(struct kobject *kobj)

@@ -633,10 +698,14 @@ static void thpsize_release(struct kobject *kobj)
      kfree(to_thpsize(kobj));
  }
 
+static void thpsize_child_release(struct kobject *kobj)
+{
+     kfree(to_thpsize_child(kobj));
+}
+
  static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
  {
      int err;
-     struct thpsize *thpsize;
      unsigned long orders;
      int order;
 

@@ -665,16 +734,14 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
              goto remove_hp_group;
      }
 
-     orders = THP_ORDERS_ALL_ANON;
+     orders = THP_ORDERS_ALL_ANON | PAGECACHE_LARGE_ORDERS;
      order = highest_order(orders);
      while (orders) {
-             thpsize = thpsize_create(order, *hugepage_kobj);
-             if (IS_ERR(thpsize)) {
+             err = thpsize_create(order, *hugepage_kobj);
+             if (err) {
                      pr_err("failed to create thpsize for order %d\n", order);
-                     err = PTR_ERR(thpsize);
                      goto remove_all;
              }
-             list_add(&thpsize->node, &thpsize_list);
              order = next_order(&orders, order);
      }
 

@@ -692,11 +759,11 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
 
  static void __init hugepage_exit_sysfs(struct kobject *hugepage_kobj)
  {
-     struct thpsize *thpsize, *tmp;
+     struct thpsize_child *child, *tmp;
 
-     list_for_each_entry_safe(thpsize, tmp, &thpsize_list, node) {
-             list_del(&thpsize->node);
-             kobject_put(&thpsize->kobj);
+     list_for_each_entry_safe(child, tmp, &thpsize_child_list, node) {
+             list_del(&child->node);
+             kobject_put(&child->kobj);
      }
 
      sysfs_remove_group(hugepage_kobj, &khugepaged_attr_group);

Quoted 68 lines · Baolin Wang

Quoted 20 lines · Ryan Roberts

Expose 3 new mTHP stats for file (pagecache) folio allocations:

/sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_alloc /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback_charge

This will provide some insight on the sizes of large folios being allocated for file-backed memory, and how often allocation is failing.

All non-order-0 (and most order-0) folio allocations are currently done through filemap_alloc_folio(), and folios are charged in a subsequent call to filemap_add_folio(). So count file_fallback when allocation fails in filemap_alloc_folio() and count file_alloc or file_fallback_charge in filemap_add_folio(), based on whether charging succeeded or not. There are some users of filemap_add_folio() that allocate their own order-0 folio by other means, so we would not count an allocation failure in this case, but we also don't care about order-0 allocations. This approach feels like it should be good enough and doesn't require any (impractically large) refactoring.

Quoted 9 lines

The existing mTHP stats interface is reused to provide consistency to users. And because we are reusing the same interface, we can reuse the same infrastructure on the kernel side. The one small wrinkle is that the set of folio sizes supported by the pagecache are not identical to those supported by anon and shmem; pagecache supports order-1, unlike anon and shmem, and the max pagecache order may be less than PMD-size (see arm64 with 64K base pages), again unlike anon and shmem. So we now create a hugepages-*kB directory for the union of the sizes supported by all 3 memory types and populate it with the relevant stats and controls.

Personally, I like the idea that can help analyze the allocation of large folios for the page cache.

However, I have a slight concern about the consistency of the interface.

For 64K, the fields layout:

├── hugepages-64kB
│   ├── enabled
│   ├── shmem_enabled
│   └── stats
│       ├── anon_fault_alloc
│       ├── anon_fault_fallback
│       ├── anon_fault_fallback_charge
│       ├── file_alloc
│       ├── file_fallback
│       ├── file_fallback_charge
│       ├── shmem_alloc
│       ├── shmem_fallback
│       ├── shmem_fallback_charge
│       ├── split
│       ├── split_deferred
│       ├── split_failed
│       ├── swpout
│       └── swpout_fallback

But for 8K (for pagecache), you removed some fields (of course, I understand why they are not supported).

├── hugepages-8kB
│   └── stats
│       ├── file_alloc
│       ├── file_fallback
│       └── file_fallback_charge

Quoted 8 lines

This might not be user-friendly for some user-space parsing tools, as they lack certain fields for the same pattern interfaces. Of course, this might not be an issue if we have clear documentation describing the differences here:)

Another possible approach is to maintain the same field layout to keep consistent, but prohibit writing to the fields that are not supported by the pagecache, and any stats read from them would be 0.

I agree that maintaining a uniform field layout, especially at the stats level, might be necessary ;)

Keeping a consistent interface could future-proof the design. It allows for the possibility that features not currently supported for 8kB pages might be enabled in the future.

Quoted 83 lines · Lance Yang

Quoted 69 lines · Baolin Wang

Quoted 21 lines · Ryan Roberts

Expose 3 new mTHP stats for file (pagecache) folio allocations:

    /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_alloc /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback_charge

This will provide some insight on the sizes of large folios being allocated for file-backed memory, and how often allocation is failing.

All non-order-0 (and most order-0) folio allocations are currently done through filemap_alloc_folio(), and folios are charged in a subsequent call to filemap_add_folio(). So count file_fallback when allocation fails in filemap_alloc_folio() and count file_alloc or file_fallback_charge in filemap_add_folio(), based on whether charging succeeded or not. There are some users of filemap_add_folio() that allocate their own order-0 folio by other means, so we would not count an allocation failure in this case, but we also don't care about order-0 allocations. This approach feels like it should be good enough and doesn't require any (impractically large) refactoring.

Quoted 9 lines

The existing mTHP stats interface is reused to provide consistency to users. And because we are reusing the same interface, we can reuse the same infrastructure on the kernel side. The one small wrinkle is that the set of folio sizes supported by the pagecache are not identical to those supported by anon and shmem; pagecache supports order-1, unlike anon and shmem, and the max pagecache order may be less than PMD-size (see arm64 with 64K base pages), again unlike anon and shmem. So we now create a hugepages-*kB directory for the union of the sizes supported by all 3 memory types and populate it with the relevant stats and controls.

Personally, I like the idea that can help analyze the allocation of large folios for the page cache.

However, I have a slight concern about the consistency of the interface.

For 64K, the fields layout:

├── hugepages-64kB
│   ├── enabled
│   ├── shmem_enabled
│   └── stats
│       ├── anon_fault_alloc
│       ├── anon_fault_fallback
│       ├── anon_fault_fallback_charge
│       ├── file_alloc
│       ├── file_fallback
│       ├── file_fallback_charge
│       ├── shmem_alloc
│       ├── shmem_fallback
│       ├── shmem_fallback_charge
│       ├── split
│       ├── split_deferred
│       ├── split_failed
│       ├── swpout
│       └── swpout_fallback

But for 8K (for pagecache), you removed some fields (of course, I understand why they are not supported).

├── hugepages-8kB
│   └── stats
│       ├── file_alloc
│       ├── file_fallback
│       └── file_fallback_charge

Quoted 8 lines

This might not be user-friendly for some user-space parsing tools, as they lack certain fields for the same pattern interfaces. Of course, this might not be an issue if we have clear documentation describing the differences here:)

Another possible approach is to maintain the same field layout to keep consistent, but prohibit writing to the fields that are not supported by the pagecache, and any stats read from them would be 0.

I agree that maintaining a uniform field layout, especially at the stats level, might be necessary ;)

Quoted 3 lines

Keeping a consistent interface could future-proof the design. It allows for the possibility that features not currently supported for 8kB pages might be enabled in the future.

I'll just note that, with shmem/file effectively being disabled for order > 11, we'll also have entries there that are effectively unused.

Good question how we want to deal with that (stats are easy, but what about when we enable something? Maybe we should document that "enabled" is only effective when supported).

Hmmmmm

Quoted 84 lines · David Hildenbrand

Quoted 83 lines · Lance Yang

Quoted 69 lines · Baolin Wang

Quoted 21 lines · Ryan Roberts

Expose 3 new mTHP stats for file (pagecache) folio allocations:

    /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_alloc /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback_charge

This will provide some insight on the sizes of large folios being allocated for file-backed memory, and how often allocation is failing.

All non-order-0 (and most order-0) folio allocations are currently done through filemap_alloc_folio(), and folios are charged in a subsequent call to filemap_add_folio(). So count file_fallback when allocation fails in filemap_alloc_folio() and count file_alloc or file_fallback_charge in filemap_add_folio(), based on whether charging succeeded or not. There are some users of filemap_add_folio() that allocate their own order-0 folio by other means, so we would not count an allocation failure in this case, but we also don't care about order-0 allocations. This approach feels like it should be good enough and doesn't require any (impractically large) refactoring.

Quoted 9 lines

The existing mTHP stats interface is reused to provide consistency to users. And because we are reusing the same interface, we can reuse the same infrastructure on the kernel side. The one small wrinkle is that the set of folio sizes supported by the pagecache are not identical to those supported by anon and shmem; pagecache supports order-1, unlike anon and shmem, and the max pagecache order may be less than PMD-size (see arm64 with 64K base pages), again unlike anon and shmem. So we now create a hugepages-*kB directory for the union of the sizes supported by all 3 memory types and populate it with the relevant stats and controls.

Personally, I like the idea that can help analyze the allocation of large folios for the page cache.

However, I have a slight concern about the consistency of the interface.

For 64K, the fields layout:

├── hugepages-64kB
│   ├── enabled
│   ├── shmem_enabled
│   └── stats
│       ├── anon_fault_alloc
│       ├── anon_fault_fallback
│       ├── anon_fault_fallback_charge
│       ├── file_alloc
│       ├── file_fallback
│       ├── file_fallback_charge
│       ├── shmem_alloc
│       ├── shmem_fallback
│       ├── shmem_fallback_charge
│       ├── split
│       ├── split_deferred
│       ├── split_failed
│       ├── swpout
│       └── swpout_fallback

But for 8K (for pagecache), you removed some fields (of course, I understand why they are not supported).

├── hugepages-8kB
│   └── stats
│       ├── file_alloc
│       ├── file_fallback
│       └── file_fallback_charge

Quoted 8 lines

This might not be user-friendly for some user-space parsing tools, as they lack certain fields for the same pattern interfaces. Of course, this might not be an issue if we have clear documentation describing the differences here:)

Another possible approach is to maintain the same field layout to keep consistent, but prohibit writing to the fields that are not supported by the pagecache, and any stats read from them would be 0.

I agree that maintaining a uniform field layout, especially at the stats level, might be necessary ;)

Quoted 6 lines

Quoted 3 lines

Keeping a consistent interface could future-proof the design. It allows for the possibility that features not currently supported for 8kB pages might be enabled in the future.

I'll just note that, with shmem/file effectively being disabled for order > 11, we'll also have entries there that are effectively unused.

Indeed, I mentioned that in the commit log :)

Quoted 4 lines

Good question how we want to deal with that (stats are easy, but what about when we enable something? Maybe we should document that "enabled" is only effective when supported).

The documentation already says "If enabling multiple hugepage sizes, the kernel will select the most appropriate enabled size for a given allocation." for anon THP (and I've added similar wording for my as-yet-unposted patch to add controls for page cache folio sizes). So I think we could easily add dummy *enabled controls for all sizes, that can be written to and read back consistently, but the kernel just ignores them when deciding what size to use. It would also simplify the code that populates the controls.

Personally though, I'm not convinced of the value of trying to make the controls for every size look identical. What's the real value to the user to pretend that they can select a size that they cannot? What happens when we inevitably want to add some new control in future which only applies to select sizes and there is no good way to fake it for the other sizes? Why can't user space just be expected to rely on the existance of the files rather than on the existance of the directories?

As always, I'll go with the majority, but just wanted to register my opinion.

Thanks,
Ryan

Quoted 3 lines

Hmmmmm

Quoted 85 lines · Ryan Roberts

Quoted 84 lines · David Hildenbrand

Quoted 83 lines · Lance Yang

Quoted 69 lines · Baolin Wang

Quoted 21 lines · Ryan Roberts

Expose 3 new mTHP stats for file (pagecache) folio allocations:

    /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_alloc /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback

/sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback_charge

This will provide some insight on the sizes of large folios being allocated for file-backed memory, and how often allocation is failing.

All non-order-0 (and most order-0) folio allocations are currently done through filemap_alloc_folio(), and folios are charged in a subsequent call to filemap_add_folio(). So count file_fallback when allocation fails in filemap_alloc_folio() and count file_alloc or file_fallback_charge in filemap_add_folio(), based on whether charging succeeded or not. There are some users of filemap_add_folio() that allocate their own order-0 folio by other means, so we would not count an allocation failure in this case, but we also don't care about order-0 allocations. This approach feels like it should be good enough and doesn't require any (impractically large) refactoring.

Quoted 9 lines

The existing mTHP stats interface is reused to provide consistency to users. And because we are reusing the same interface, we can reuse the same infrastructure on the kernel side. The one small wrinkle is that the set of folio sizes supported by the pagecache are not identical to those supported by anon and shmem; pagecache supports order-1, unlike anon and shmem, and the max pagecache order may be less than PMD-size (see arm64 with 64K base pages), again unlike anon and shmem. So we now create a hugepages-*kB directory for the union of the sizes supported by all 3 memory types and populate it with the relevant stats and controls.

Personally, I like the idea that can help analyze the allocation of large folios for the page cache.

However, I have a slight concern about the consistency of the interface.

For 64K, the fields layout:

├── hugepages-64kB
│   ├── enabled
│   ├── shmem_enabled
│   └── stats
│       ├── anon_fault_alloc
│       ├── anon_fault_fallback
│       ├── anon_fault_fallback_charge
│       ├── file_alloc
│       ├── file_fallback
│       ├── file_fallback_charge
│       ├── shmem_alloc
│       ├── shmem_fallback
│       ├── shmem_fallback_charge
│       ├── split
│       ├── split_deferred
│       ├── split_failed
│       ├── swpout
│       └── swpout_fallback

But for 8K (for pagecache), you removed some fields (of course, I understand why they are not supported).

├── hugepages-8kB
│   └── stats
│       ├── file_alloc
│       ├── file_fallback
│       └── file_fallback_charge

Quoted 8 lines

This might not be user-friendly for some user-space parsing tools, as they lack certain fields for the same pattern interfaces. Of course, this might not be an issue if we have clear documentation describing the differences here:)

Another possible approach is to maintain the same field layout to keep consistent, but prohibit writing to the fields that are not supported by the pagecache, and any stats read from them would be 0.

I agree that maintaining a uniform field layout, especially at the stats level, might be necessary ;)

Quoted 8 lines

Quoted 6 lines

Quoted 3 lines

Keeping a consistent interface could future-proof the design. It allows for the possibility that features not currently supported for 8kB pages might be enabled in the future.

I'll just note that, with shmem/file effectively being disabled for order > 11, we'll also have entries there that are effectively unused.

Indeed, I mentioned that in the commit log :)

Quoted 22 lines

Quoted 5 lines

Good question how we want to deal with that (stats are easy, but what about when we enable something? Maybe we should document that "enabled" is only effective when supported).

The documentation already says "If enabling multiple hugepage sizes, the kernel will select the most appropriate enabled size for a given allocation." for anon THP (and I've added similar wording for my as-yet-unposted patch to add controls for page cache folio sizes). So I think we could easily add dummy *enabled controls for all sizes, that can be written to and read back consistently, but the kernel just ignores them when deciding what size to use. It would also simplify the code that populates the controls.

Personally though, I'm not convinced of the value of trying to make the controls for every size look identical. What's the real value to the user to pretend that they can select a size that they cannot? What happens when we inevitably want to add some new control in future which only applies to select sizes and there is no good way to fake it for the other sizes? Why can't user space just be expected to rely on the existance of the files rather than on the existance of the directories?

Quoted 1 line

As always, I'll go with the majority, but just wanted to register my opinion.

Should I assume from the lack of reply on this that everyone else is in favour of adding dummy controls so that all sizes have the same set of controls? If I don't hear anything further, I'll post v2 with dummry controls today or tomorrow.

Quoted 93 lines · Ryan Roberts

Quoted 92 lines · Ryan Roberts

Quoted 90 lines · David Hildenbrand

Quoted 80 lines · Lance Yang

Quoted 69 lines · Baolin Wang

Quoted 21 lines · Ryan Roberts

Expose 3 new mTHP stats for file (pagecache) folio allocations:

     /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_alloc /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback /sys/kernel/mm/transparent_hugepage/hugepages-*kB/stats/file_fallback_charge

This will provide some insight on the sizes of large folios being allocated for file-backed memory, and how often allocation is failing.

All non-order-0 (and most order-0) folio allocations are currently done through filemap_alloc_folio(), and folios are charged in a subsequent call to filemap_add_folio(). So count file_fallback when allocation fails in filemap_alloc_folio() and count file_alloc or file_fallback_charge in filemap_add_folio(), based on whether charging succeeded or not. There are some users of filemap_add_folio() that allocate their own order-0 folio by other means, so we would not count an allocation failure in this case, but we also don't care about order-0 allocations. This approach feels like it should be good enough and doesn't require any (impractically large) refactoring.

Quoted 9 lines

The existing mTHP stats interface is reused to provide consistency to users. And because we are reusing the same interface, we can reuse the same infrastructure on the kernel side. The one small wrinkle is that the set of folio sizes supported by the pagecache are not identical to those supported by anon and shmem; pagecache supports order-1, unlike anon and shmem, and the max pagecache order may be less than PMD-size (see arm64 with 64K base pages), again unlike anon and shmem. So we now create a hugepages-*kB directory for the union of the sizes supported by all 3 memory types and populate it with the relevant stats and controls.

Personally, I like the idea that can help analyze the allocation of large folios for the page cache.

However, I have a slight concern about the consistency of the interface.

For 64K, the fields layout:

├── hugepages-64kB
│   ├── enabled
│   ├── shmem_enabled
│   └── stats
│       ├── anon_fault_alloc
│       ├── anon_fault_fallback
│       ├── anon_fault_fallback_charge
│       ├── file_alloc
│       ├── file_fallback
│       ├── file_fallback_charge
│       ├── shmem_alloc
│       ├── shmem_fallback
│       ├── shmem_fallback_charge
│       ├── split
│       ├── split_deferred
│       ├── split_failed
│       ├── swpout
│       └── swpout_fallback

But for 8K (for pagecache), you removed some fields (of course, I understand why they are not supported).

├── hugepages-8kB
│   └── stats
│       ├── file_alloc
│       ├── file_fallback
│       └── file_fallback_charge

Quoted 8 lines

This might not be user-friendly for some user-space parsing tools, as they lack certain fields for the same pattern interfaces. Of course, this might not be an issue if we have clear documentation describing the differences here:)

Another possible approach is to maintain the same field layout to keep consistent, but prohibit writing to the fields that are not supported by the pagecache, and any stats read from them would be 0.

Quoted 6 lines

I agree that maintaining a uniform field layout, especially at the stats level, might be necessary ;)

Keeping a consistent interface could future-proof the design. It allows for the possibility that features not currently supported for 8kB pages might be enabled in the future.

I'll just note that, with shmem/file effectively being disabled for order > 11, we'll also have entries there that are effectively unused.

Quoted 1 line

Quoted 1 line

Indeed, I mentioned that in the commit log :)

Well, I think it's more extreme than what you mentioned.

For example, shmem_enable on arm64 with 64k is now effectively non-functional. Just like it will be for other orders in the anon-shmem case when the order exceeds MAX_PAGECACHE_ORDER.

Quoted 26 lines

Quoted 25 lines

Quoted 7 lines

Good question how we want to deal with that (stats are easy, but what about when
we enable something? Maybe we should document that "enabled" is only effective when supported).

The documentation already says "If enabling multiple hugepage sizes, the kernel will select the most appropriate enabled size for a given allocation." for anon THP (and I've added similar wording for my as-yet-unposted patch to add controls for page cache folio sizes). So I think we could easily add dummy *enabled controls for all sizes, that can be written to and read back consistently, but the kernel just ignores them when deciding what size to use. It would also simplify the code that populates the controls.

Personally though, I'm not convinced of the value of trying to make the controls for every size look identical. What's the real value to the user to pretend that they can select a size that they cannot? What happens when we inevitably want to add some new control in future which only applies to select sizes and there is no good way to fake it for the other sizes? Why can't user space just be expected to rely on the existance of the files rather than on the existance of the directories?

As always, I'll go with the majority, but just wanted to register my opinion.

Quoted 3 lines

Should I assume from the lack of reply on this that everyone else is in favour of adding dummy controls so that all sizes have the same set of controls? If I don't hear anything further, I'll post v2 with dummry controls today or tomorrow.

Sorry, busy with other stuff.

Indicating only what really exists sounds cleaner. But I wonder how we would want to handle in general orders that are effectively non-existant?

Quoted 4 lines

Sorry, busy with other stuff.

Indicating only what really exists sounds cleaner. But I wonder how we would want to handle in general orders that are effectively non-existant?

I'm not following your distinction between orders that don't "really exist" and orders that are "effectively non-existant".

I guess the real supported orders are:

anon:
min order: 2
max order: PMD_ORDER
anon-shmem:
min order: 1
max order: MAX_PAGECACHE_ORDER
tmpfs-shmem:
min order: PMD_ORDER <= 11 ? PMD_ORDER : NONE max order: PMD_ORDER <= 11 ? PMD_ORDER : NONE
file:
min order: 1
max order: MAX_PAGECACHE_ORDER

But today, controls and stats are exposed for:

anon:
min order: 2
max order: PMD_ORDER
anon-shmem:
min order: 2
max order: PMD_ORDER
tmpfs-shmem:
min order: PMD_ORDER
max order: PMD_ORDER
file:
min order: Nothing yet (this patch proposes 1)
max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)

So I think there is definitely a bug for shmem where the minimum order control should be order-1 but its currently order-2.

I also wonder about PUD-order for DAX? We don't currently have a stat/control. If we wanted to add it in future, if we take the "expose all stats/controls for all orders" approach, we would end up extending all the way to PUD-order and all the orders between PMD and PUD would be dummy for all memory types. That really starts to feel odd, so I still favour only populating what's really supported.

I propose to fix shmem (extend down to 1, stop at MAX_PAGECACHE_ORDER) and continue with the approach of "indicating only what really exists" for v2.

Shout if you disagree.

Quoted 7 lines

Quoted 4 lines

Sorry, busy with other stuff.

Indicating only what really exists sounds cleaner. But I wonder how we would want to handle in general orders that are effectively non-existant?

I'm not following your distinction between orders that don't "really exist" and orders that are "effectively non-existant".

I'm questioning whether there should be a distinction at all. We should just hide what is either non-existant (not implemented) or non-functional.

Quoted 4 lines

I guess the real supported orders are:

Quoted 12 lines

   anon:
     min order: 2
     max order: PMD_ORDER
   anon-shmem:
     min order: 1
     max order: MAX_PAGECACHE_ORDER tmpfs-shmem:
     min order: PMD_ORDER <= 11 ? PMD_ORDER : NONE max order: PMD_ORDER <= 11 ? PMD_ORDER : NONE file:
     min order: 1
     max order: MAX_PAGECACHE_ORDER

That's my understanding. But not sure about anon-shmem really supporting order-1, maybe we do.

Quoted 17 lines

But today, controls and stats are exposed for:

   anon:
     min order: 2
     max order: PMD_ORDER
   anon-shmem:
     min order: 2
     max order: PMD_ORDER
   tmpfs-shmem:
     min order: PMD_ORDER
     max order: PMD_ORDER
   file:
     min order: Nothing yet (this patch proposes 1)
     max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)

Quoted 2 lines

So I think there is definitely a bug for shmem where the minimum order control should be order-1 but its currently order-2.

Maybe, did not play with that yet. Likely order-1 will work. (although probably of questionable use :) )

Quoted 7 lines

I also wonder about PUD-order for DAX? We don't currently have a stat/control. If we wanted to add it in future, if we take the "expose all stats/controls for all orders" approach, we would end up extending all the way to PUD-order and all the orders between PMD and PUD would be dummy for all memory types. That really starts to feel odd, so I still favour only populating what's really supported.

I would go further and say that calling the fsdax thing a THP is borderline wrong and we should not expose any new toggles for it that way.

It really behaves much more like hugetlb folios that can be PTE-mapped ... we cannot split these things, and they are not allocated from the buddy. So I wouldn't worry about fsdax for now.

fsdax support for compound pages (now large folios) probably never should have been glued to any THP toggle.

Quoted 6 lines

I propose to fix shmem (extend down to 1, stop at MAX_PAGECACHE_ORDER) and continue with the approach of "indicating only what really exists" for v2.

Shout if you disagree.

Makes sense.

Quoted 15 lines

Quoted 14 lines

I guess the real supported orders are:

   anon:
     min order: 2
     max order: PMD_ORDER
   anon-shmem:
     min order: 1
     max order: MAX_PAGECACHE_ORDER tmpfs-shmem:
     min order: PMD_ORDER <= 11 ? PMD_ORDER : NONE max order: PMD_ORDER <= 11 ? PMD_ORDER : NONE file:
     min order: 1
     max order: MAX_PAGECACHE_ORDER

Quoted 2 lines

That's my understanding. But not sure about anon-shmem really supporting order-1, maybe we do.

Oh, I thought we only had the restriction for anon folios now (due to deferred split queue), so assumed it would just work. With Gavin's THP_ORDERS_ALL_FILE_DEFAULT change, that certainly implies that shmem must support order-1. If it doesn't then we we might want to tidy that further.

Baolin, perhaps you can confirm either way?

Quoted 21 lines

Quoted 20 lines

But today, controls and stats are exposed for:

   anon:
     min order: 2
     max order: PMD_ORDER
   anon-shmem:
     min order: 2
     max order: PMD_ORDER
   tmpfs-shmem:
     min order: PMD_ORDER
     max order: PMD_ORDER
   file:
     min order: Nothing yet (this patch proposes 1)
     max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)

So I think there is definitely a bug for shmem where the minimum order control should be order-1 but its currently order-2.

Quoted 2 lines

Maybe, did not play with that yet. Likely order-1 will work. (although probably of questionable use :) )

You might have to expand on why its of "questionable use". I'd assume it has the same amount of value as using order-1 for regular page cache pages? i.e. half the number of objects to manage for the same amount of memory.

Quoted 16 lines

Quoted 8 lines

I also wonder about PUD-order for DAX? We don't currently have a stat/control. If we wanted to add it in future, if we take the "expose all stats/controls for all orders" approach, we would end up extending all the way to PUD-order and all the orders between PMD and PUD would be dummy for all memory types. That really starts to feel odd, so I still favour only populating what's really supported.

I would go further and say that calling the fsdax thing a THP is borderline wrong and we should not expose any new toggles for it that way.

It really behaves much more like hugetlb folios that can be PTE-mapped ... we cannot split these things, and they are not allocated from the buddy. So I wouldn't worry about fsdax for now.

Quoted 2 lines

fsdax support for compound pages (now large folios) probably never should have been glued to any THP toggle.

Yeah fair enough. I wasn't really arguing for adding any dax controls; I was just trying to think of examples as to why adding dummy controls might be a bad idea.

Quoted 8 lines

Quoted 6 lines

I propose to fix shmem (extend down to 1, stop at MAX_PAGECACHE_ORDER) and continue with the approach of "indicating only what really exists" for v2.

Shout if you disagree.

Makes sense.

Excellent. I posted v2, which has these changes, yesterday afternoon. :)

Quoted 18 lines

Quoted 17 lines

Quoted 14 lines

I guess the real supported orders are:

    anon:
      min order: 2
      max order: PMD_ORDER
    anon-shmem:
      min order: 1
      max order: MAX_PAGECACHE_ORDER tmpfs-shmem:
      min order: PMD_ORDER <= 11 ? PMD_ORDER : NONE max order: PMD_ORDER <= 11 ? PMD_ORDER : NONE file:
      min order: 1
      max order: MAX_PAGECACHE_ORDER

That's my understanding. But not sure about anon-shmem really supporting order-1, maybe we do.

Quoted 6 lines

Oh, I thought we only had the restriction for anon folios now (due to deferred split queue), so assumed it would just work. With Gavin's THP_ORDERS_ALL_FILE_DEFAULT change, that certainly implies that shmem must support order-1. If it doesn't then we we might want to tidy that further.

Baolin, perhaps you can confirm either way?

Currently there would not have been a way to enable it, right? (maybe I'm wrong)

Quoted 22 lines

Quoted 22 lines

Quoted 21 lines

But today, controls and stats are exposed for:

    anon:
      min order: 2
      max order: PMD_ORDER
    anon-shmem:
      min order: 2
      max order: PMD_ORDER
    tmpfs-shmem:
      min order: PMD_ORDER
      max order: PMD_ORDER
    file:
      min order: Nothing yet (this patch proposes 1)
      max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)

So I think there is definitely a bug for shmem where the minimum order control should be order-1 but its currently order-2.

Quoted 6 lines

Quoted 2 lines

Maybe, did not play with that yet. Likely order-1 will work. (although probably of questionable use :) )

You might have to expand on why its of "questionable use". I'd assume it has the same amount of value as using order-1 for regular page cache pages? i.e. half the number of objects to manage for the same amount of memory.

order-1 was recently added for the pagecache to get some device setups running (IIRC, where we cannot use order-0, because device blocksize > PAGE_SIZE).

You might be right about "half the number of objects", but likely just going for order-2, order-3, order-4 ... for shmem might be even better. And simply falling back to order-0 when you cannot get the larger orders.

I could have sworn you mentioned something like that in your "configurable orders for pagecache" RFC that I only briefly skimmed so far :P

... only enabling "order-1" and none of the other orders for shmem sounds rather "interesting".

But yeah, maybe there is valid use for it, so I'm all for allowing it if it can be done.

Quoted 17 lines

Quoted 17 lines

Quoted 9 lines

I also wonder about PUD-order for DAX? We don't currently have a stat/control. If we wanted to add it in future, if we take the "expose all stats/controls for all orders" approach, we would end up extending all the way to PUD-order and all
the orders between PMD and PUD would be dummy for all memory types. That really starts to feel odd, so I still favour only populating what's really supported.

I would go further and say that calling the fsdax thing a THP is borderline wrong and we should not expose any new toggles for it that way.

It really behaves much more like hugetlb folios that can be PTE-mapped ... we cannot split these things, and they are not allocated from the buddy. So I wouldn't worry about fsdax for now.

Quoted 6 lines

Quoted 2 lines

fsdax support for compound pages (now large folios) probably never should have been glued to any THP toggle.

Yeah fair enough. I wasn't really arguing for adding any dax controls; I was just trying to think of examples as to why adding dummy controls might be a bad idea.

Yes.

Quoted 5 lines

Quoted 5 lines

Quoted 5 lines

I propose to fix shmem (extend down to 1, stop at MAX_PAGECACHE_ORDER) and continue with the approach of "indicating only what really exists" for v2.

Quoted 5 lines

Quoted 3 lines

Quoted 1 line

Shout if you disagree.

Makes sense.

Excellent. I posted v2, which has these changes, yesterday afternoon. :)

Yes, still digging through mails ... in-between having roughly 1000 meetings a day :)

Quoted 29 lines

Quoted 19 lines

Quoted 19 lines

Quoted 18 lines

But today, controls and stats are exposed for:

    anon:
      min order: 2
      max order: PMD_ORDER
    anon-shmem:
      min order: 2
      max order: PMD_ORDER
    tmpfs-shmem:
      min order: PMD_ORDER
      max order: PMD_ORDER
    file:
      min order: Nothing yet (this patch proposes 1)
      max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)

So I think there is definitely a bug for shmem where the minimum order control should be order-1 but its currently order-2.

Quoted 6 lines

Quoted 2 lines

Maybe, did not play with that yet. Likely order-1 will work. (although probably of questionable use :) )

You might have to expand on why its of "questionable use". I'd assume it has the same amount of value as using order-1 for regular page cache pages? i.e. half the number of objects to manage for the same amount of memory.

order-1 was recently added for the pagecache to get some device setups running (IIRC, where we cannot use order-0, because device blocksize > PAGE_SIZE).

Quoted 3 lines

You might be right about "half the number of objects", but likely just going for order-2, order-3, order-4 ... for shmem might be even better. And simply falling back to order-0 when you cannot get the larger orders.

Sure, but then you're into the territory of baking in policy. Remember that originally I was only interested in 64K but the concensus was to expose all the sizes. Same argument applies to 8K; expose it and let others decide policy.

Quoted 36 lines

Quoted 35 lines

Quoted 20 lines

Quoted 20 lines

Quoted 19 lines

But today, controls and stats are exposed for:

     anon:
       min order: 2
       max order: PMD_ORDER
     anon-shmem:
       min order: 2
       max order: PMD_ORDER
     tmpfs-shmem:
       min order: PMD_ORDER
       max order: PMD_ORDER
     file:
       min order: Nothing yet (this patch proposes 1)
       max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)

So I think there is definitely a bug for shmem where the minimum order control
should be order-1 but its currently order-2.

Quoted 8 lines

Quoted 3 lines

Maybe, did not play with that yet. Likely order-1 will work. (although probably
of questionable use :) )

You might have to expand on why its of "questionable use". I'd assume it has the
same amount of value as using order-1 for regular page cache pages? i.e. half the number of objects to manage for the same amount of memory.

order-1 was recently added for the pagecache to get some device setups running (IIRC, where we cannot use order-0, because device blocksize > PAGE_SIZE).

You might be right about "half the number of objects", but likely just going for order-2, order-3, order-4 ... for shmem might be even better. And simply falling back to order-0 when you cannot get the larger orders.

Quoted 3 lines

Sure, but then you're into the territory of baking in policy. Remember that originally I was only interested in 64K but the concensus was to expose all the sizes. Same argument applies to 8K; expose it and let others decide policy.

I don't disagree. The point I'm trying to make is that there was so far there was no strong evidence that it is really required. Support for the pagecache had a different motivation for these special devices.

But again, I agree that we should just make it consistent and allow for it. :)

Quoted 40 lines · David Hildenbrand

Quoted 39 lines

Quoted 29 lines

Quoted 20 lines

Quoted 20 lines

Quoted 19 lines

But today, controls and stats are exposed for:

     anon:
       min order: 2
       max order: PMD_ORDER
     anon-shmem:
       min order: 2
       max order: PMD_ORDER
     tmpfs-shmem:
       min order: PMD_ORDER
       max order: PMD_ORDER
     file:
       min order: Nothing yet (this patch proposes 1)
       max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)

So I think there is definitely a bug for shmem where the minimum order control
should be order-1 but its currently order-2.

Quoted 8 lines

Quoted 3 lines

Maybe, did not play with that yet. Likely order-1 will work. (although probably
of questionable use :) )

You might have to expand on why its of "questionable use". I'd assume it has the
same amount of value as using order-1 for regular page cache pages? i.e. half the number of objects to manage for the same amount of memory.

Quoted 6 lines

order-1 was recently added for the pagecache to get some device setups running (IIRC, where we cannot use order-0, because device blocksize > PAGE_SIZE).

You might be right about "half the number of objects", but likely just going for order-2, order-3, order-4 ... for shmem might be even better. And simply falling back to order-0 when you cannot get the larger orders.

Sure, but then you're into the territory of baking in policy. Remember that originally I was only interested in 64K but the concensus was to expose all the sizes. Same argument applies to 8K; expose it and let others decide policy.

Quoted 3 lines

I don't disagree. The point I'm trying to make is that there was so far there was no strong evidence that it is really required. Support for the pagecache had a different motivation for these special devices.

Sure, but there was no clear need for anon mTHP orders other than order-2 and order-4 (for arm64's HPA and contpte, respectively), but we still chose to expose all the others.

Quoted 43 lines · Ryan Roberts

Quoted 42 lines · David Hildenbrand

Quoted 41 lines

Quoted 29 lines

Quoted 20 lines

Quoted 20 lines

Quoted 19 lines

But today, controls and stats are exposed for:

      anon:
        min order: 2
        max order: PMD_ORDER
      anon-shmem:
        min order: 2
        max order: PMD_ORDER
      tmpfs-shmem:
        min order: PMD_ORDER
        max order: PMD_ORDER
      file:
        min order: Nothing yet (this patch proposes 1)
        max order: Nothing yet (this patch proposes MAX_PAGECACHE_ORDER)

So I think there is definitely a bug for shmem where the minimum order control
should be order-1 but its currently order-2.

Quoted 8 lines

Quoted 3 lines

Maybe, did not play with that yet. Likely order-1 will work. (although probably
of questionable use :) )

You might have to expand on why its of "questionable use". I'd assume it has the
same amount of value as using order-1 for regular page cache pages? i.e. half the number of objects to manage for the same amount of memory.

Quoted 8 lines

order-1 was recently added for the pagecache to get some device setups running (IIRC, where we cannot use order-0, because device blocksize > PAGE_SIZE).

You might be right about "half the number of objects", but likely just going for
order-2, order-3, order-4 ... for shmem might be even better. And simply falling
back to order-0 when you cannot get the larger orders.

Sure, but then you're into the territory of baking in policy. Remember that originally I was only interested in 64K but the concensus was to expose all the sizes. Same argument applies to 8K; expose it and let others decide policy.

Quoted 7 lines

Quoted 3 lines

I don't disagree. The point I'm trying to make is that there was so far there was no strong evidence that it is really required. Support for the pagecache had a different motivation for these special devices.

Sure, but there was no clear need for anon mTHP orders other than order-2 and order-4 (for arm64's HPA and contpte, respectively), but we still chose to expose all the others.

order-2 and order-3 are valuable for AMD EPYC (depending on the generation 16 vs. 32 KiB coalescing).

But in general, at least for me, it's easier to argue why larger orders make more sense than very tiny ones.

For example, order-5 can be mapped using cont-pte as well and you get roughly half the memory allocation+page fault overhead compared to order-4.

order-1 ? No TLB optimization at least on any current HW I know.

But I believe we're in violent agreement here :)

Quoted 67 lines

+static int thpsize_create(int order, struct kobject *parent)
  {
      unsigned long size = (PAGE_SIZE << order) / SZ_1K;
+    struct thpsize_child *stats;
      struct thpsize *thpsize;
      int ret;
  +    /*
+     * Each child object (currently only "stats" directory) holds a +     * reference to the top-level thpsize object, so we can drop our ref to +     * the top-level once stats is setup. Then we just need to drop a +     * reference on any children to clean everything up. We can't just use +     * the attr group name for the stats subdirectory because there may be +     * multiple attribute groups to populate inside stats and overlaying +     * using the name property isn't supported in that way; each attr group +     * name, if provided, must be unique in the parent directory. +     */
+
      thpsize = kzalloc(sizeof(*thpsize), GFP_KERNEL);
-    if (!thpsize)
-        return ERR_PTR(-ENOMEM);
+    if (!thpsize) {
+        ret = -ENOMEM;
+        goto err;
+    }
+    thpsize->order = order;
        ret = kobject_init_and_add(&thpsize->kobj, &thpsize_ktype, parent, "hugepages-%lukB", size);
      if (ret) {
          kfree(thpsize);
-        return ERR_PTR(ret);
+        goto err;
      }
  -    ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group); -    if (ret) {
+    stats = kzalloc(sizeof(*stats), GFP_KERNEL);
+    if (!stats) {
          kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        ret = -ENOMEM;
+        goto err;
      }
  -    ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group); +    ret = kobject_init_and_add(&stats->kobj, &thpsize_child_ktype, +                   &thpsize->kobj, "stats");
+    kobject_put(&thpsize->kobj);
      if (ret) {
-        kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        kfree(stats);
+        goto err;
      }
  -    thpsize->order = order;
-    return thpsize;
+    if (BIT(order) & THP_ORDERS_ALL_ANON) {
+        ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group); +        if (ret)
+            goto err_put;

mm/huge_memory.c +10 -0

+
+        ret = sysfs_create_group(&stats->kobj, &stats_attr_group);
+        if (ret)
+            goto err_put;
+    }
+
+    if (BIT(order) & PAGECACHE_LARGE_ORDERS) {
+        ret = sysfs_create_group(&stats->kobj, &file_stats_attr_group);
+        if (ret)
+            goto err_put;

mm/huge_memory.c static struct attribute *stats_attrs[] = { · line 690

Quoted 5 lines

+    }

mm/huge_memory.c +4 -0

+
+    list_add(&stats->node, &thpsize_child_list);
+    return 0;
+err_put:

IIUC, I think you should call 'sysfs_remove_group' to remove the group before putting the kobject.

Quoted 5 lines

+    kobject_put(&stats->kobj);
+err:
+    return ret;
  }

[...]

Quoted 67 lines

Quoted 67 lines

mm/huge_memory.c +41 -8

+static int thpsize_create(int order, struct kobject *parent)
   {
       unsigned long size = (PAGE_SIZE << order) / SZ_1K;
+    struct thpsize_child *stats;
       struct thpsize *thpsize;
       int ret;
   +    /*
+     * Each child object (currently only "stats" directory) holds a
+     * reference to the top-level thpsize object, so we can drop our ref to
+     * the top-level once stats is setup. Then we just need to drop a
+     * reference on any children to clean everything up. We can't just use
+     * the attr group name for the stats subdirectory because there may be
+     * multiple attribute groups to populate inside stats and overlaying
+     * using the name property isn't supported in that way; each attr group
+     * name, if provided, must be unique in the parent directory.
+     */
+
       thpsize = kzalloc(sizeof(*thpsize), GFP_KERNEL);
-    if (!thpsize)
-        return ERR_PTR(-ENOMEM);
+    if (!thpsize) {
+        ret = -ENOMEM;
+        goto err;
+    }
+    thpsize->order = order;
         ret = kobject_init_and_add(&thpsize->kobj, &thpsize_ktype, parent,
                      "hugepages-%lukB", size);
       if (ret) {
           kfree(thpsize);
-        return ERR_PTR(ret);
+        goto err;
       }
   -    ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
-    if (ret) {
+    stats = kzalloc(sizeof(*stats), GFP_KERNEL);
+    if (!stats) {
           kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        ret = -ENOMEM;
+        goto err;
       }
   -    ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group);
+    ret = kobject_init_and_add(&stats->kobj, &thpsize_child_ktype,
+                   &thpsize->kobj, "stats");
+    kobject_put(&thpsize->kobj);
       if (ret) {
-        kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        kfree(stats);
+        goto err;
       }
   -    thpsize->order = order;
-    return thpsize;
+    if (BIT(order) & THP_ORDERS_ALL_ANON) {
+        ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
+        if (ret)
+            goto err_put;
+
+        ret = sysfs_create_group(&stats->kobj, &stats_attr_group);
+        if (ret)
+            goto err_put;
+    }
+
+    if (BIT(order) & PAGECACHE_LARGE_ORDERS) {
+        ret = sysfs_create_group(&stats->kobj, &file_stats_attr_group);
+        if (ret)
+            goto err_put;

Quoted 8 lines

mm/huge_memory.c static struct attribute *stats_attrs[] = { · line 690

Quoted 5 lines

+    }

mm/huge_memory.c +4 -0

+
+    list_add(&stats->node, &thpsize_child_list);
+    return 0;
+err_put:

IIUC, I think you should call 'sysfs_remove_group' to remove the group before putting the kobject.

Are you sure about that? As I understood it, sysfs_create_group() was conceptually modifying the state of the kobj, so when the kobj gets destroyed, all its state is tidied up. __kobject_del() (called on the last kobject_put()) calls sysfs_remove_groups() and tidies up the sysfs state as far as I can see?

Quoted 78 lines · Ryan Roberts

[...]

Quoted 67 lines

Quoted 67 lines

+static int thpsize_create(int order, struct kobject *parent)
   {
       unsigned long size = (PAGE_SIZE << order) / SZ_1K;
+    struct thpsize_child *stats;
       struct thpsize *thpsize;
       int ret;
   +    /*
+     * Each child object (currently only "stats" directory) holds a +     * reference to the top-level thpsize object, so we can drop our ref to +     * the top-level once stats is setup. Then we just need to drop a +     * reference on any children to clean everything up. We can't just use +     * the attr group name for the stats subdirectory because there may be +     * multiple attribute groups to populate inside stats and overlaying +     * using the name property isn't supported in that way; each attr group +     * name, if provided, must be unique in the parent directory. +     */
+
       thpsize = kzalloc(sizeof(*thpsize), GFP_KERNEL);
-    if (!thpsize)
-        return ERR_PTR(-ENOMEM);
+    if (!thpsize) {
+        ret = -ENOMEM;
+        goto err;
+    }
+    thpsize->order = order;
         ret = kobject_init_and_add(&thpsize->kobj, &thpsize_ktype, parent, "hugepages-%lukB", size);
       if (ret) {
           kfree(thpsize);
-        return ERR_PTR(ret);
+        goto err;
       }
   -    ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group); -    if (ret) {
+    stats = kzalloc(sizeof(*stats), GFP_KERNEL);
+    if (!stats) {
           kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        ret = -ENOMEM;
+        goto err;
       }
   -    ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group); +    ret = kobject_init_and_add(&stats->kobj, &thpsize_child_ktype, +                   &thpsize->kobj, "stats");
+    kobject_put(&thpsize->kobj);
       if (ret) {
-        kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        kfree(stats);
+        goto err;
       }
   -    thpsize->order = order;
-    return thpsize;
+    if (BIT(order) & THP_ORDERS_ALL_ANON) {
+        ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group); +        if (ret)
+            goto err_put;

mm/huge_memory.c +10 -0

+
+        ret = sysfs_create_group(&stats->kobj, &stats_attr_group);
+        if (ret)
+            goto err_put;
+    }
+
+    if (BIT(order) & PAGECACHE_LARGE_ORDERS) {
+        ret = sysfs_create_group(&stats->kobj, &file_stats_attr_group);
+        if (ret)
+            goto err_put;

Quoted 8 lines

mm/huge_memory.c static struct attribute *stats_attrs[] = { · line 690

Quoted 5 lines

+    }

mm/huge_memory.c +4 -0

+
+    list_add(&stats->node, &thpsize_child_list);
+    return 0;
+err_put:

IIUC, I think you should call 'sysfs_remove_group' to remove the group before putting the kobject.

Quoted 4 lines

Are you sure about that? As I understood it, sysfs_create_group() was conceptually modifying the state of the kobj, so when the kobj gets destroyed, all its state is tidied up. __kobject_del() (called on the last kobject_put()) calls sysfs_remove_groups() and tidies up the sysfs state as far as I can see?

IIUC, __kobject_del() only removes the ktype defaut groups by 'sysfs_remove_groups(kobj, ktype->default_groups)', but your created groups are not added into the ktype->default_groups. That means you should mannuly remove them, or am I miss something?

Quoted 86 lines · Baolin Wang

Quoted 78 lines · Ryan Roberts

[...]

Quoted 67 lines

Quoted 67 lines

mm/huge_memory.c +41 -8

+static int thpsize_create(int order, struct kobject *parent)
    {
        unsigned long size = (PAGE_SIZE << order) / SZ_1K;
+    struct thpsize_child *stats;
        struct thpsize *thpsize;
        int ret;
    +    /*
+     * Each child object (currently only "stats" directory) holds a
+     * reference to the top-level thpsize object, so we can drop our ref to
+     * the top-level once stats is setup. Then we just need to drop a
+     * reference on any children to clean everything up. We can't just use
+     * the attr group name for the stats subdirectory because there may be
+     * multiple attribute groups to populate inside stats and overlaying
+     * using the name property isn't supported in that way; each attr group
+     * name, if provided, must be unique in the parent directory.
+     */
+
        thpsize = kzalloc(sizeof(*thpsize), GFP_KERNEL);
-    if (!thpsize)
-        return ERR_PTR(-ENOMEM);
+    if (!thpsize) {
+        ret = -ENOMEM;
+        goto err;
+    }
+    thpsize->order = order;
          ret = kobject_init_and_add(&thpsize->kobj, &thpsize_ktype, parent,
                       "hugepages-%lukB", size);
        if (ret) {
            kfree(thpsize);
-        return ERR_PTR(ret);
+        goto err;
        }
    -    ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
-    if (ret) {
+    stats = kzalloc(sizeof(*stats), GFP_KERNEL);
+    if (!stats) {
            kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        ret = -ENOMEM;
+        goto err;
        }
    -    ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group);
+    ret = kobject_init_and_add(&stats->kobj, &thpsize_child_ktype,
+                   &thpsize->kobj, "stats");
+    kobject_put(&thpsize->kobj);
        if (ret) {
-        kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        kfree(stats);
+        goto err;
        }
    -    thpsize->order = order;
-    return thpsize;
+    if (BIT(order) & THP_ORDERS_ALL_ANON) {
+        ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
+        if (ret)
+            goto err_put;
+
+        ret = sysfs_create_group(&stats->kobj, &stats_attr_group);
+        if (ret)
+            goto err_put;
+    }
+
+    if (BIT(order) & PAGECACHE_LARGE_ORDERS) {
+        ret = sysfs_create_group(&stats->kobj, &file_stats_attr_group);
+        if (ret)
+            goto err_put;

Quoted 8 lines

mm/huge_memory.c static struct attribute *stats_attrs[] = { · line 690

Quoted 5 lines

+    }

mm/huge_memory.c +4 -0

+
+    list_add(&stats->node, &thpsize_child_list);
+    return 0;
+err_put:

IIUC, I think you should call 'sysfs_remove_group' to remove the group before putting the kobject.

Quoted 4 lines

Are you sure about that? As I understood it, sysfs_create_group() was conceptually modifying the state of the kobj, so when the kobj gets destroyed, all its state is tidied up. __kobject_del() (called on the last kobject_put()) calls sysfs_remove_groups() and tidies up the sysfs state as far as I can see?

Quoted 4 lines

IIUC, __kobject_del() only removes the ktype defaut groups by 'sysfs_remove_groups(kobj, ktype->default_groups)', but your created groups are not added into the ktype->default_groups. That means you should mannuly remove them, or am I miss something?

That was also putting doubt in my mind. But the sample at samples/kobject/kobject-example.c does not call sysfs_remove_group(). It just calls sysfs_create_group() in example_init() and calls kobject_put() in example_exit(). So I think that's the correct pattern.

Looking at the code more closely, sysfs_create_group() just creates files for each of the attributes in the group. __kobject_del() calls sysfs_remove_dir(), who's comment states "we remove any files in the directory before we remove the directory" so I'm pretty sure sysfs_remove_group() is not required.

Quoted 97 lines · Ryan Roberts

Quoted 86 lines · Baolin Wang

Quoted 78 lines · Ryan Roberts

[...]

Quoted 67 lines

Quoted 67 lines

+static int thpsize_create(int order, struct kobject *parent)
    {
        unsigned long size = (PAGE_SIZE << order) / SZ_1K;
+    struct thpsize_child *stats;
        struct thpsize *thpsize;
        int ret;
    +    /*
+     * Each child object (currently only "stats" directory) holds a +     * reference to the top-level thpsize object, so we can drop our ref to +     * the top-level once stats is setup. Then we just need to drop a +     * reference on any children to clean everything up. We can't just use +     * the attr group name for the stats subdirectory because there may be +     * multiple attribute groups to populate inside stats and overlaying +     * using the name property isn't supported in that way; each attr group +     * name, if provided, must be unique in the parent directory. +     */
+
        thpsize = kzalloc(sizeof(*thpsize), GFP_KERNEL);
-    if (!thpsize)
-        return ERR_PTR(-ENOMEM);
+    if (!thpsize) {
+        ret = -ENOMEM;
+        goto err;
+    }
+    thpsize->order = order;
          ret = kobject_init_and_add(&thpsize->kobj, &thpsize_ktype, parent, "hugepages-%lukB", size);
        if (ret) {
            kfree(thpsize);
-        return ERR_PTR(ret);
+        goto err;
        }
    -    ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group); -    if (ret) {
+    stats = kzalloc(sizeof(*stats), GFP_KERNEL);
+    if (!stats) {
            kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        ret = -ENOMEM;
+        goto err;
        }
    -    ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group); +    ret = kobject_init_and_add(&stats->kobj, &thpsize_child_ktype, +                   &thpsize->kobj, "stats");
+    kobject_put(&thpsize->kobj);
        if (ret) {
-        kobject_put(&thpsize->kobj);
-        return ERR_PTR(ret);
+        kfree(stats);
+        goto err;
        }
    -    thpsize->order = order;
-    return thpsize;
+    if (BIT(order) & THP_ORDERS_ALL_ANON) {
+        ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group); +        if (ret)
+            goto err_put;

mm/huge_memory.c +10 -0

+
+        ret = sysfs_create_group(&stats->kobj, &stats_attr_group);
+        if (ret)
+            goto err_put;
+    }
+
+    if (BIT(order) & PAGECACHE_LARGE_ORDERS) {
+        ret = sysfs_create_group(&stats->kobj, &file_stats_attr_group);
+        if (ret)
+            goto err_put;

Quoted 8 lines

mm/huge_memory.c static struct attribute *stats_attrs[] = { · line 690

Quoted 5 lines

+    }

mm/huge_memory.c +4 -0

+
+    list_add(&stats->node, &thpsize_child_list);
+    return 0;
+err_put:

IIUC, I think you should call 'sysfs_remove_group' to remove the group before putting the kobject.

Quoted 4 lines

Are you sure about that? As I understood it, sysfs_create_group() was conceptually modifying the state of the kobj, so when the kobj gets destroyed, all its state is tidied up. __kobject_del() (called on the last kobject_put()) calls sysfs_remove_groups() and tidies up the sysfs state as far as I can see?

Quoted 4 lines

IIUC, __kobject_del() only removes the ktype defaut groups by 'sysfs_remove_groups(kobj, ktype->default_groups)', but your created groups are not added into the ktype->default_groups. That means you should mannuly remove them, or am I miss something?

That was also putting doubt in my mind. But the sample at samples/kobject/kobject-example.c does not call sysfs_remove_group(). It just calls sysfs_create_group() in example_init() and calls kobject_put() in example_exit(). So I think that's the correct pattern.

Quoted 4 lines

Looking at the code more closely, sysfs_create_group() just creates files for each of the attributes in the group. __kobject_del() calls sysfs_remove_dir(), who's comment states "we remove any files in the directory before we remove the directory" so I'm pretty sure sysfs_remove_group() is not required.

Thanks for the explanation, and I think you are right after checking the code again. Sorry for the noise.

Hi Ryan,

kernel test robot noticed the following build errors:

[auto build test ERROR on akpm-mm/mm-everything]
[also build test ERROR on next-20240712]
[cannot apply to linus/master v6.10-rc7]
[If your patch is applied to the wrong git tree, kindly drop us a note. And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Ryan-Roberts/mm-Cleanup-count_mthp_stat-definition/20240711-154455
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20240711072929.3590000-3-ryan.roberts%40arm.com
patch subject: [PATCH v1 2/2] mm: mTHP stats for pagecache folio allocations
config: arm64-defconfig (https://download.01.org/0day-ci/archive/20240713/[email protected]/config)
compiler: aarch64-linux-gcc (GCC) 14.1.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240713/[email protected]/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <[email protected]>
| Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/

All errors (new ones prefixed by >>, old ones prefixed by <<):

Quoted 1 line

Quoted 1 line

ERROR: modpost: "mthp_stats" [fs/btrfs/btrfs.ko] undefined!