add mTHP support for anonymous shmem

Hi Baolin,

kernel test robot noticed the following build warnings:

[auto build test WARNING on akpm-mm/mm-everything]
[also build test WARNING on linus/master v6.10-rc2 next-20240604] [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/Baolin-Wang/mm-memory-extend-finish_fault-to-support-large-folio/20240604-182028
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/bee11bfd9157e60aaea6db033a4af7c13c982c82.1717495894.git.baolin.wang%40linux.alibaba.com
patch subject: [PATCH v4 1/6] mm: memory: extend finish_fault() to support large folio
config: openrisc-allnoconfig (https://download.01.org/0day-ci/archive/20240604/[email protected]/config)
compiler: or1k-linux-gcc (GCC) 13.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240604/[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 warnings (new ones prefixed by >>):

mm/memory.c: In function 'finish_fault':

Quoted 1 line

Quoted 1 line

mm/memory.c:4838:29: warning: unused variable 'i' [-Wunused-variable]

     4838 |         int type, nr_pages, i;
          |                             ^

To support the use of mTHP with anonymous shmem, add a new sysfs interface 'shmem_enabled' in the '/sys/kernel/mm/transparent_hugepage/hugepages-kB/' directory for each mTHP to control whether shmem is enabled for that mTHP, with a value similar to the top level 'shmem_enabled', which can be set to:
"always", "inherit (to inherit the top level setting)", "within_size", "advise", "never". An 'inherit' option is added to ensure compatibility with these global settings, and the options 'force' and 'deny' are dropped, which are rather testing artifacts from the old ages.

By default, PMD-sized hugepages have enabled="inherit" and all other hugepage sizes have enabled="never" for '/sys/kernel/mm/transparent_hugepage/hugepages-xxkB/shmem_enabled'.

In addition, if top level value is 'force', then only PMD-sized hugepages have enabled="inherit", otherwise configuration will be failed and vice versa. That means now we will avoid using non-PMD sized THP to override the global huge allocation.

Signed-off-by Baolin Wang <[email protected]>

---
 Documentation/admin-guide/mm/transhuge.rst | 23 ++++++
 include/linux/huge_mm.h                    | 10 +++
 mm/huge_memory.c                           | 11 +--
 mm/shmem.c                                 | 96 ++++++++++++++++++++++
 4 files changed, 132 insertions(+), 8 deletions(-)

Documentation/admin-guide/mm/transhuge.rst +23 -0

index d414d3f5592a..b76d15e408b3 100644
@@ -332,6 +332,29 @@ deny
 force
     Force the huge option on for all - very useful for testing;
 
+Shmem can also use "multi-size THP" (mTHP) by adding a new sysfs knob to control
+mTHP allocation: '/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled',
+and its value for each mTHP is essentially consistent with the global setting.
+An 'inherit' option is added to ensure compatibility with these global settings.
+Conversely, the options 'force' and 'deny' are dropped, which are rather testing
+artifacts from the old ages.
+always
+    Attempt to allocate <size> huge pages every time we need a new page;
+
+inherit
+    Inherit the top-level "shmem_enabled" value. By default, PMD-sized hugepages
+    have enabled="inherit" and all other hugepage sizes have enabled="never";
+
+never
+    Do not allocate <size> huge pages;
+
+within_size
+    Only allocate <size> huge page if it will be fully within i_size.
+    Also respect fadvise()/madvise() hints;
+
+advise
+    Only allocate <size> huge pages if requested with fadvise()/madvise();
+
 Need of application restart
 ===========================
 

include/linux/huge_mm.h +10 -0

index 020e2344eb86..fac21548c5de 100644
@@ -6,6 +6,7 @@
 #include <linux/mm_types.h>
 
 #include <linux/fs.h> /* only for vma_is_dax() */
+#include <linux/kobject.h>
 
 vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf);
 int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,

@@ -63,6 +64,7 @@ ssize_t single_hugepage_flag_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf,
 				  enum transparent_hugepage_flag flag);
 extern struct kobj_attribute shmem_enabled_attr;
+extern struct kobj_attribute thpsize_shmem_enabled_attr;
 
 /*
  * Mask of all large folio orders supported for anonymous THP; all orders up to

@@ -265,6 +267,14 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
 	return __thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
 }
 
+struct thpsize {
+	struct kobject kobj;
+	struct list_head node;
+	int order;
+};
+
+#define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
+
 enum mthp_stat_item {
 	MTHP_STAT_ANON_FAULT_ALLOC,
 	MTHP_STAT_ANON_FAULT_FALLBACK,

mm/huge_memory.c +3 -8

index 8e49f402d7c7..1360a1903b66 100644
@@ -449,14 +449,6 @@ static void thpsize_release(struct kobject *kobj);
 static DEFINE_SPINLOCK(huge_anon_orders_lock);
 static LIST_HEAD(thpsize_list);
 
-struct thpsize {
-	struct kobject kobj;
-	struct list_head node;
-	int order;
-};
-
-#define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
-
 static ssize_t thpsize_enabled_show(struct kobject *kobj,
 				    struct kobj_attribute *attr, char *buf)
 {

@@ -517,6 +509,9 @@ static struct kobj_attribute thpsize_enabled_attr =
 
 static struct attribute *thpsize_attrs[] = {
 	&thpsize_enabled_attr.attr,
+#ifdef CONFIG_SHMEM
+	&thpsize_shmem_enabled_attr.attr,
+#endif
 	NULL,
 };
 

mm/shmem.c +0 -0

index ae358efc397a..643ff7516b4d 100644

mm/shmem.c +89 -0

+
 #ifdef CONFIG_TMPFS
 static unsigned long shmem_default_max_blocks(void)
 {

@@ -4672,6 +4680,12 @@ void __init shmem_init(void)
 		SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge;
 	else
 		shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */
+
+	/*
+	 * Default to setting PMD-sized THP to inherit the global setting and
+	 * disable all other multi-size THPs, when anonymous shmem uses mTHP.
+	 */
+	huge_anon_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
 #endif
 	return;
 

@@ -4731,6 +4745,11 @@ static ssize_t shmem_enabled_store(struct kobject *kobj,
 			huge != SHMEM_HUGE_NEVER && huge != SHMEM_HUGE_DENY)
 		return -EINVAL;
 
+	/* Do not override huge allocation policy with non-PMD sized mTHP */
+	if (huge == SHMEM_HUGE_FORCE &&
+	    huge_anon_shmem_orders_inherit != BIT(HPAGE_PMD_ORDER))
+		return -EINVAL;
+
 	shmem_huge = huge;
 	if (shmem_huge > SHMEM_HUGE_DENY)
 		SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge;

@@ -4738,6 +4757,83 @@ static ssize_t shmem_enabled_store(struct kobject *kobj,
 }
 
 struct kobj_attribute shmem_enabled_attr = __ATTR_RW(shmem_enabled);
+
+static ssize_t thpsize_shmem_enabled_show(struct kobject *kobj,
+					  struct kobj_attribute *attr, char *buf)
+{
+	int order = to_thpsize(kobj)->order;
+	const char *output;
+
+	if (test_bit(order, &huge_anon_shmem_orders_always))
+		output = "[always] inherit within_size advise never";
+	else if (test_bit(order, &huge_anon_shmem_orders_inherit))
+		output = "always [inherit] within_size advise never";
+	else if (test_bit(order, &huge_anon_shmem_orders_within_size))
+		output = "always inherit [within_size] advise never";
+	else if (test_bit(order, &huge_anon_shmem_orders_madvise))
+		output = "always inherit within_size [advise] never";
+	else
+		output = "always inherit within_size advise [never]";
+
+	return sysfs_emit(buf, "%s\n", output);
+}
+
+static ssize_t thpsize_shmem_enabled_store(struct kobject *kobj,
+					   struct kobj_attribute *attr,
+					   const char *buf, size_t count)
+{
+	int order = to_thpsize(kobj)->order;
+	ssize_t ret = count;
+
+	if (sysfs_streq(buf, "always")) {
+		spin_lock(&huge_anon_shmem_orders_lock);
+		clear_bit(order, &huge_anon_shmem_orders_inherit);
+		clear_bit(order, &huge_anon_shmem_orders_madvise);
+		clear_bit(order, &huge_anon_shmem_orders_within_size);
+		set_bit(order, &huge_anon_shmem_orders_always);
+		spin_unlock(&huge_anon_shmem_orders_lock);
+	} else if (sysfs_streq(buf, "inherit")) {
+		/* Do not override huge allocation policy with non-PMD sized mTHP */
+		if (shmem_huge == SHMEM_HUGE_FORCE &&
+		    order != HPAGE_PMD_ORDER)
+			return -EINVAL;
+
+		spin_lock(&huge_anon_shmem_orders_lock);
+		clear_bit(order, &huge_anon_shmem_orders_always);
+		clear_bit(order, &huge_anon_shmem_orders_madvise);
+		clear_bit(order, &huge_anon_shmem_orders_within_size);
+		set_bit(order, &huge_anon_shmem_orders_inherit);
+		spin_unlock(&huge_anon_shmem_orders_lock);
+	} else if (sysfs_streq(buf, "within_size")) {
+		spin_lock(&huge_anon_shmem_orders_lock);
+		clear_bit(order, &huge_anon_shmem_orders_always);
+		clear_bit(order, &huge_anon_shmem_orders_inherit);
+		clear_bit(order, &huge_anon_shmem_orders_madvise);
+		set_bit(order, &huge_anon_shmem_orders_within_size);
+		spin_unlock(&huge_anon_shmem_orders_lock);
+	} else if (sysfs_streq(buf, "madvise")) {
+		spin_lock(&huge_anon_shmem_orders_lock);
+		clear_bit(order, &huge_anon_shmem_orders_always);
+		clear_bit(order, &huge_anon_shmem_orders_inherit);
+		clear_bit(order, &huge_anon_shmem_orders_within_size);
+		set_bit(order, &huge_anon_shmem_orders_madvise);
+		spin_unlock(&huge_anon_shmem_orders_lock);
+	} else if (sysfs_streq(buf, "never")) {
+		spin_lock(&huge_anon_shmem_orders_lock);
+		clear_bit(order, &huge_anon_shmem_orders_always);
+		clear_bit(order, &huge_anon_shmem_orders_inherit);
+		clear_bit(order, &huge_anon_shmem_orders_within_size);
+		clear_bit(order, &huge_anon_shmem_orders_madvise);
+		spin_unlock(&huge_anon_shmem_orders_lock);
+	} else {
+		ret = -EINVAL;
+	}
+
+	return ret;
+}
+
+struct kobj_attribute thpsize_shmem_enabled_attr =
+	__ATTR(shmem_enabled, 0644, thpsize_shmem_enabled_show, thpsize_shmem_enabled_store);
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE && CONFIG_SYSFS */
 
 #else /* !CONFIG_SHMEM */

Signature · 1 line

2.39.3

Hi Baolin,

Quoted 131 lines · Baolin Wang

To support the use of mTHP with anonymous shmem, add a new sysfs interface 'shmem_enabled' in the '/sys/kernel/mm/transparent_hugepage/hugepages-kB/' directory for each mTHP to control whether shmem is enabled for that mTHP, with a value similar to the top level 'shmem_enabled', which can be set to:
"always", "inherit (to inherit the top level setting)", "within_size", "advise", "never". An 'inherit' option is added to ensure compatibility with these global settings, and the options 'force' and 'deny' are dropped, which are rather testing artifacts from the old ages.

By default, PMD-sized hugepages have enabled="inherit" and all other hugepage sizes have enabled="never" for '/sys/kernel/mm/transparent_hugepage/hugepages-xxkB/shmem_enabled'.

In addition, if top level value is 'force', then only PMD-sized hugepages have enabled="inherit", otherwise configuration will be failed and vice versa. That means now we will avoid using non-PMD sized THP to override the global huge allocation.

Signed-off-by Baolin Wang <[email protected]>

---
  Documentation/admin-guide/mm/transhuge.rst | 23 ++++++
  include/linux/huge_mm.h                    | 10 +++
  mm/huge_memory.c                           | 11 +--
  mm/shmem.c                                 | 96 ++++++++++++++++++++++
  4 files changed, 132 insertions(+), 8 deletions(-)

Documentation/admin-guide/mm/transhuge.rst +23 -0

index d414d3f5592a..b76d15e408b3 100644
@@ -332,6 +332,29 @@ deny
  force
      Force the huge option on for all - very useful for testing;
 
+Shmem can also use "multi-size THP" (mTHP) by adding a new sysfs knob to control
+mTHP allocation: '/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled',
+and its value for each mTHP is essentially consistent with the global setting.
+An 'inherit' option is added to ensure compatibility with these global settings.
+Conversely, the options 'force' and 'deny' are dropped, which are rather testing
+artifacts from the old ages.
+always
+    Attempt to allocate <size> huge pages every time we need a new page;
+
+inherit
+    Inherit the top-level "shmem_enabled" value. By default, PMD-sized hugepages
+    have enabled="inherit" and all other hugepage sizes have enabled="never";
+
+never
+    Do not allocate <size> huge pages;
+
+within_size
+    Only allocate <size> huge page if it will be fully within i_size.
+    Also respect fadvise()/madvise() hints;
+
+advise
+    Only allocate <size> huge pages if requested with fadvise()/madvise();
+
  Need of application restart
  ===========================
 

include/linux/huge_mm.h +10 -0

index 020e2344eb86..fac21548c5de 100644
@@ -6,6 +6,7 @@
  #include <linux/mm_types.h>
 
  #include <linux/fs.h> /* only for vma_is_dax() */
+#include <linux/kobject.h>
 
  vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf);
  int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,

@@ -63,6 +64,7 @@ ssize_t single_hugepage_flag_show(struct kobject *kobj,
  				  struct kobj_attribute *attr, char *buf,
  				  enum transparent_hugepage_flag flag);
  extern struct kobj_attribute shmem_enabled_attr;
+extern struct kobj_attribute thpsize_shmem_enabled_attr;
 
  /*
   * Mask of all large folio orders supported for anonymous THP; all orders up to

@@ -265,6 +267,14 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
  	return __thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
  }
 
+struct thpsize {
+	struct kobject kobj;
+	struct list_head node;
+	int order;
+};
+
+#define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
+
  enum mthp_stat_item {
  	MTHP_STAT_ANON_FAULT_ALLOC,
  	MTHP_STAT_ANON_FAULT_FALLBACK,

mm/huge_memory.c +3 -8

index 8e49f402d7c7..1360a1903b66 100644
@@ -449,14 +449,6 @@ static void thpsize_release(struct kobject *kobj);
  static DEFINE_SPINLOCK(huge_anon_orders_lock);
  static LIST_HEAD(thpsize_list);
 
-struct thpsize {
-	struct kobject kobj;
-	struct list_head node;
-	int order;
-};
-
-#define to_thpsize(kobj) container_of(kobj, struct thpsize, kobj)
-
  static ssize_t thpsize_enabled_show(struct kobject *kobj,
  				    struct kobj_attribute *attr, char *buf)
  {

@@ -517,6 +509,9 @@ static struct kobj_attribute thpsize_enabled_attr =
 
  static struct attribute *thpsize_attrs[] = {
  	&thpsize_enabled_attr.attr,
+#ifdef CONFIG_SHMEM
+	&thpsize_shmem_enabled_attr.attr,
+#endif
  	NULL,
  };
 

mm/shmem.c +0 -0

index ae358efc397a..643ff7516b4d 100644
@@ -131,6 +131,14 @@ struct shmem_options {
  #define SHMEM_SEEN_QUOTA 32
  };
 

mm/shmem.c struct shmem_options { · line 140

Quoted 7 lines

mm/shmem.c +7 -0

+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static unsigned long huge_anon_shmem_orders_always __read_mostly;
+static unsigned long huge_anon_shmem_orders_madvise __read_mostly;
+static unsigned long huge_anon_shmem_orders_inherit __read_mostly;
+static unsigned long huge_anon_shmem_orders_within_size __read_mostly;
+static DEFINE_SPINLOCK(huge_anon_shmem_orders_lock);
+#endif

Since we are also applying the new sysfs knob controls to tmpfs and anon mm, should we rename this to get rid of the anon prefix?

Commit 19eaf44954df adds multi-size THP (mTHP) for anonymous pages, that can allow THP to be configured through the sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.

However, the anonymous shmem will ignore the anonymous mTHP rule configured through the sysfs interface, and can only use the PMD-mapped THP, that is not reasonable. Users expect to apply the mTHP rule for all anonymous pages, including the anonymous shmem, in order to enjoy the benefits of mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc. In addition, the mTHP interfaces can be extended to support all shmem/tmpfs scenarios in the future, especially for the shmem mmap() case.

The primary strategy is similar to supporting anonymous mTHP. Introduce a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled', which can have almost the same values as the top-level '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new additional "inherit" option and dropping the testing options 'force' and 'deny'. By default all sizes will be set to "never" except PMD size, which is set to "inherit". This ensures backward compatibility with the anonymous shmem enabled of the top level, meanwhile also allows independent control of anonymous shmem enabled for each mTHP.

Signed-off-by Baolin Wang <[email protected]>

---
 include/linux/huge_mm.h |  10 +++
 mm/shmem.c              | 187 +++++++++++++++++++++++++++++++++-------
 2 files changed, 167 insertions(+), 30 deletions(-)

include/linux/huge_mm.h +10 -0

index fac21548c5de..909cfc67521d 100644
@@ -575,6 +575,16 @@ static inline bool thp_migration_supported(void)
 {
 	return false;
 }
+
+static inline int highest_order(unsigned long orders)
+{
+	return 0;
+}
+
+static inline int next_order(unsigned long *orders, int prev)
+{
+	return 0;
+}
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 static inline int split_folio_to_list_to_order(struct folio *folio,

mm/shmem.c +0 -0

index 643ff7516b4d..9a8533482208 100644

mm/shmem.c +43 -0

+	if ((vm_flags & VM_NOHUGEPAGE) ||
+	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
+		return 0;
+
+	/* If the hardware/firmware marked hugepage support disabled. */
+	if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_UNSUPPORTED))
+		return 0;
+
+	/*
+	 * Following the 'deny' semantics of the top level, force the huge
+	 * option off from all mounts.
+	 */
+	if (shmem_huge == SHMEM_HUGE_DENY)
+		return 0;
+
+	/*
+	 * Only allow inherit orders if the top-level value is 'force', which
+	 * means non-PMD sized THP can not override 'huge' mount option now.
+	 */
+	if (shmem_huge == SHMEM_HUGE_FORCE)
+		return READ_ONCE(huge_anon_shmem_orders_inherit);
+
+	/* Allow mTHP that will be fully within i_size. */
+	order = highest_order(within_size_orders);
+	while (within_size_orders) {
+		index = round_up(index + 1, order);
+		i_size = round_up(i_size_read(inode), PAGE_SIZE);
+		if (i_size >> PAGE_SHIFT >= index) {
+			mask |= within_size_orders;
+			break;
+		}
+
+		order = next_order(&within_size_orders, order);
+	}
+
+	if (vm_flags & VM_HUGEPAGE)
+		mask |= READ_ONCE(huge_anon_shmem_orders_madvise);
+
+	if (global_huge)
+		mask |= READ_ONCE(huge_anon_shmem_orders_inherit);
+
+	return orders & mask;
+}

mm/shmem.c +38 -5

+	if (!orders)
+		return 0;
+
+	/* Find the highest order that can add into the page cache */
+	order = highest_order(orders);
+	while (orders) {
+		pages = 1UL << order;
+		index = round_down(index, pages);
+		if (!xa_find(&mapping->i_pages, &index,
+			     index + pages - 1, XA_PRESENT))
+			break;
+		order = next_order(&orders, order);
+	}
+
+	return orders;
+}
+#else
+static unsigned long anon_shmem_allowable_huge_orders(struct inode *inode,
+				struct vm_area_struct *vma, pgoff_t index,
+				bool global_huge)
+{
+	return 0;
+}
+
+static unsigned long anon_shmem_suitable_orders(struct inode *inode, struct vm_fault *vmf,
+					struct address_space *mapping, pgoff_t index,
+					unsigned long orders)
+{
+	return 0;
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+
 static struct folio *shmem_alloc_folio(gfp_t gfp, int order,
 		struct shmem_inode_info *info, pgoff_t index)
 {

@@ -1625,38 +1726,55 @@ static struct folio *shmem_alloc_folio(gfp_t gfp, int order,
 	return folio;
 }
 
-static struct folio *shmem_alloc_and_add_folio(gfp_t gfp,
-		struct inode *inode, pgoff_t index,
-		struct mm_struct *fault_mm, bool huge)
+static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
+		gfp_t gfp, struct inode *inode, pgoff_t index,
+		struct mm_struct *fault_mm, unsigned long orders)
 {
 	struct address_space *mapping = inode->i_mapping;
 	struct shmem_inode_info *info = SHMEM_I(inode);
-	struct folio *folio;
+	struct vm_area_struct *vma = vmf ? vmf->vma : NULL;
+	unsigned long suitable_orders = 0;
+	struct folio *folio = NULL;
 	long pages;
-	int error;

mm/shmem.c +34 -15

+		if (vma && vma_is_anon_shmem(vma)) {
+			suitable_orders = anon_shmem_suitable_orders(inode, vmf,
+							mapping, index, orders);
+		} else if (orders & BIT(HPAGE_PMD_ORDER)) {
+			pages = HPAGE_PMD_NR;
+			suitable_orders = BIT(HPAGE_PMD_ORDER);
+			index = round_down(index, HPAGE_PMD_NR);
 
-		/*
-		 * Check for conflict before waiting on a huge allocation.
-		 * Conflict might be that a huge page has just been allocated
-		 * and added to page cache by a racing thread, or that there
-		 * is already at least one small page in the huge extent.
-		 * Be careful to retry when appropriate, but not forever!
-		 * Elsewhere -EEXIST would be the right code, but not here.
-		 */
-		if (xa_find(&mapping->i_pages, &index,
-				index + HPAGE_PMD_NR - 1, XA_PRESENT))
-			return ERR_PTR(-E2BIG);
+			/*
+			 * Check for conflict before waiting on a huge allocation.
+			 * Conflict might be that a huge page has just been allocated
+			 * and added to page cache by a racing thread, or that there
+			 * is already at least one small page in the huge extent.
+			 * Be careful to retry when appropriate, but not forever!
+			 * Elsewhere -EEXIST would be the right code, but not here.
+			 */
+			if (xa_find(&mapping->i_pages, &index,
+				    index + HPAGE_PMD_NR - 1, XA_PRESENT))
+				return ERR_PTR(-E2BIG);
+		}
 
-		folio = shmem_alloc_folio(gfp, HPAGE_PMD_ORDER, info, index);
-		if (!folio && pages == HPAGE_PMD_NR)
-			count_vm_event(THP_FILE_FALLBACK);
+		order = highest_order(suitable_orders);
+		while (suitable_orders) {
+			pages = 1UL << order;
+			index = round_down(index, pages);
+			folio = shmem_alloc_folio(gfp, order, info, index);
+			if (folio)
+				goto allocated;
+
+			if (pages == HPAGE_PMD_NR)
+				count_vm_event(THP_FILE_FALLBACK);
+			order = next_order(&suitable_orders, order);
+		}
 	} else {
 		pages = 1;
 		folio = shmem_alloc_folio(gfp, 0, info, index);

@@ -1664,6 +1782,7 @@ static struct folio *shmem_alloc_and_add_folio(gfp_t gfp,
 	if (!folio)
 		return ERR_PTR(-ENOMEM);
 
+allocated:
 	__folio_set_locked(folio);
 	__folio_set_swapbacked(folio);
 

@@ -1958,7 +2077,8 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	struct mm_struct *fault_mm;
 	struct folio *folio;
 	int error;
-	bool alloced;
+	bool alloced, huge;
+	unsigned long orders = 0;
 
 	if (WARN_ON_ONCE(!shmem_mapping(inode->i_mapping)))
 		return -EINVAL;

mm/shmem.c +4 -4

 		huge_gfp = limit_gfp_mask(huge_gfp, gfp);
-		folio = shmem_alloc_and_add_folio(huge_gfp,
-				inode, index, fault_mm, true);
+		folio = shmem_alloc_and_add_folio(vmf, huge_gfp,
+				inode, index, fault_mm, orders);
 		if (!IS_ERR(folio)) {
 			if (folio_test_pmd_mappable(folio))
 				count_vm_event(THP_FILE_ALLOC);

@@ -2047,7 +2174,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 			goto repeat;
 	}
 
-	folio = shmem_alloc_and_add_folio(gfp, inode, index, fault_mm, false);
+	folio = shmem_alloc_and_add_folio(vmf, gfp, inode, index, fault_mm, 0);
 	if (IS_ERR(folio)) {
 		error = PTR_ERR(folio);
 		if (error == -EEXIST)

@@ -2058,7 +2185,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 
 alloced:
 	alloced = true;
-	if (folio_test_pmd_mappable(folio) &&
+	if (folio_test_large(folio) &&
 	    DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) <
 					folio_next_index(folio) - 1) {
 		struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);

Signature · 1 line

2.39.3

Quoted 59 lines · Baolin Wang

Commit 19eaf44954df adds multi-size THP (mTHP) for anonymous pages, that can allow THP to be configured through the sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.

However, the anonymous shmem will ignore the anonymous mTHP rule configured through the sysfs interface, and can only use the PMD-mapped THP, that is not reasonable. Users expect to apply the mTHP rule for all anonymous pages, including the anonymous shmem, in order to enjoy the benefits of mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc. In addition, the mTHP interfaces can be extended to support all shmem/tmpfs scenarios in the future, especially for the shmem mmap() case.

The primary strategy is similar to supporting anonymous mTHP. Introduce a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled', which can have almost the same values as the top-level '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new additional "inherit" option and dropping the testing options 'force' and 'deny'. By default all sizes will be set to "never" except PMD size, which is set to "inherit". This ensures backward compatibility with the anonymous shmem enabled of the top level, meanwhile also allows independent control of anonymous shmem enabled for each mTHP.

Signed-off-by Baolin Wang <[email protected]>

---
  include/linux/huge_mm.h |  10 +++
  mm/shmem.c              | 187 +++++++++++++++++++++++++++++++++-------
  2 files changed, 167 insertions(+), 30 deletions(-)

include/linux/huge_mm.h +10 -0

index fac21548c5de..909cfc67521d 100644
@@ -575,6 +575,16 @@ static inline bool thp_migration_supported(void)
  {
  	return false;
  }
+
+static inline int highest_order(unsigned long orders)
+{
+	return 0;
+}
+
+static inline int next_order(unsigned long *orders, int prev)
+{
+	return 0;
+}
  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
  static inline int split_folio_to_list_to_order(struct folio *folio,

mm/shmem.c +0 -0

index 643ff7516b4d..9a8533482208 100644
@@ -1611,6 +1611,107 @@ static gfp_t limit_gfp_mask(gfp_t huge_gfp, gfp_t limit_gfp)
  	return result;
  }
 

mm/shmem.c static gfp_t limit_gfp_mask(gfp_t huge_gfp, gfp_t limit_gfp) · line 1615

Quoted 2 lines

mm/shmem.c +2 -0

+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static unsigned long anon_shmem_allowable_huge_orders(struct inode *inode,

We want to get mTHP orders as well for tmpfs so, could we make this to work for > both paths? If true, I'd remove the anon prefix.

mm/shmem.c static gfp_t limit_gfp_mask(gfp_t huge_gfp, gfp_t limit_gfp) · line 1617

Quoted 2 lines

mm/shmem.c +2 -0

+				struct vm_area_struct *vma, pgoff_t index,
+				bool global_huge)

Why did you rename 'huge' variable to 'global_huge'? We were using 'huge' in shmem_alloc_and_add_folio() before this commit. I guess it's just odd to me this var rename without seen any name conflict inside it.

Quoted 7 lines

mm/shmem.c +7 -0

+{
+	unsigned long mask = READ_ONCE(huge_anon_shmem_orders_always);
+	unsigned long within_size_orders = READ_ONCE(huge_anon_shmem_orders_within_size);
+	unsigned long vm_flags = vma->vm_flags;
+	/*
+	 * Check all the (large) orders below HPAGE_PMD_ORDER + 1 that
+	 * are enabled for this vma.

mm/shmem.c static gfp_t limit_gfp_mask(gfp_t huge_gfp, gfp_t limit_gfp) · line 1629

Quoted 5 lines

mm/shmem.c +5 -0

+	 */
+	unsigned long orders = BIT(PMD_ORDER + 1) - 1;
+	loff_t i_size;
+	int order;
+

We can start the mm anon path here but we should exclude the ones that do not apply for tmpfs.

Quoted 44 lines

mm/shmem.c +43 -0

+	if ((vm_flags & VM_NOHUGEPAGE) ||
+	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
+		return 0;
+
+	/* If the hardware/firmware marked hugepage support disabled. */
+	if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_UNSUPPORTED))
+		return 0;
+
+	/*
+	 * Following the 'deny' semantics of the top level, force the huge
+	 * option off from all mounts.
+	 */
+	if (shmem_huge == SHMEM_HUGE_DENY)
+		return 0;
+
+	/*
+	 * Only allow inherit orders if the top-level value is 'force', which
+	 * means non-PMD sized THP can not override 'huge' mount option now.
+	 */
+	if (shmem_huge == SHMEM_HUGE_FORCE)
+		return READ_ONCE(huge_anon_shmem_orders_inherit);
+
+	/* Allow mTHP that will be fully within i_size. */
+	order = highest_order(within_size_orders);
+	while (within_size_orders) {
+		index = round_up(index + 1, order);
+		i_size = round_up(i_size_read(inode), PAGE_SIZE);
+		if (i_size >> PAGE_SHIFT >= index) {
+			mask |= within_size_orders;
+			break;
+		}
+
+		order = next_order(&within_size_orders, order);
+	}
+
+	if (vm_flags & VM_HUGEPAGE)
+		mask |= READ_ONCE(huge_anon_shmem_orders_madvise);
+
+	if (global_huge)
+		mask |= READ_ONCE(huge_anon_shmem_orders_inherit);
+
+	return orders & mask;
+}

mm/shmem.c static gfp_t limit_gfp_mask(gfp_t huge_gfp, gfp_t limit_gfp) · line 1682

Quoted 10 lines

mm/shmem.c +10 -0

+
+static unsigned long anon_shmem_suitable_orders(struct inode *inode, struct vm_fault *vmf,
+					struct address_space *mapping, pgoff_t index,
+					unsigned long orders)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	unsigned long pages;
+	int order;
+
+	orders = thp_vma_suitable_orders(vma, vmf->address, orders);

This won't apply to tmpfs. I'm thinking if we can apply shmem_mapping_size_order() [1] here for tmpfs path so we have the same suitable orders for both paths.

[1] https://lore.kernel.org/all/v5acpezkt4ml3j3ufmbgnq5b335envea7xfobvowtaetvbt3an@v3pfkwly5jh2/#t

Quoted 61 lines

mm/shmem.c +40 -6

+	if (!orders)
+		return 0;
+
+	/* Find the highest order that can add into the page cache */
+	order = highest_order(orders);
+	while (orders) {
+		pages = 1UL << order;
+		index = round_down(index, pages);
+		if (!xa_find(&mapping->i_pages, &index,
+			     index + pages - 1, XA_PRESENT))
+			break;
+		order = next_order(&orders, order);
+	}
+
+	return orders;
+}
+#else
+static unsigned long anon_shmem_allowable_huge_orders(struct inode *inode,
+				struct vm_area_struct *vma, pgoff_t index,
+				bool global_huge)
+{
+	return 0;
+}
+
+static unsigned long anon_shmem_suitable_orders(struct inode *inode, struct vm_fault *vmf,
+					struct address_space *mapping, pgoff_t index,
+					unsigned long orders)
+{
+	return 0;
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+
  static struct folio *shmem_alloc_folio(gfp_t gfp, int order,
  		struct shmem_inode_info *info, pgoff_t index)
  {

@@ -1625,38 +1726,55 @@ static struct folio *shmem_alloc_folio(gfp_t gfp, int order,
  	return folio;
  }
 
-static struct folio *shmem_alloc_and_add_folio(gfp_t gfp,
-		struct inode *inode, pgoff_t index,
-		struct mm_struct *fault_mm, bool huge)
+static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
+		gfp_t gfp, struct inode *inode, pgoff_t index,
+		struct mm_struct *fault_mm, unsigned long orders)
  {
  	struct address_space *mapping = inode->i_mapping;
  	struct shmem_inode_info *info = SHMEM_I(inode);
-	struct folio *folio;
+	struct vm_area_struct *vma = vmf ? vmf->vma : NULL;
+	unsigned long suitable_orders = 0;
+	struct folio *folio = NULL;
  	long pages;
-	int error;
+	int error, order;
 
  	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
-		huge = false;
+		orders = 0;
 

mm/shmem.c static struct folio *shmem_alloc_folio(gfp_t gfp, int order, · line 1744

Quoted 4 lines

mm/shmem.c +1 -3

-	if (huge) {
-		pages = HPAGE_PMD_NR;
-		index = round_down(index, HPAGE_PMD_NR);
+	if (orders > 0) {

Can we get rid of this condition if we handle all allowable orders in 'orders'? Including order-0 and PMD-order. I agree, we do not need the huge flag anymore since you have handled all cases in shmem_allowable_huge_orders().

Quoted 73 lines

mm/shmem.c +34 -15

+		if (vma && vma_is_anon_shmem(vma)) {
+			suitable_orders = anon_shmem_suitable_orders(inode, vmf,
+							mapping, index, orders);
+		} else if (orders & BIT(HPAGE_PMD_ORDER)) {
+			pages = HPAGE_PMD_NR;
+			suitable_orders = BIT(HPAGE_PMD_ORDER);
+			index = round_down(index, HPAGE_PMD_NR);
 
-		/*
-		 * Check for conflict before waiting on a huge allocation.
-		 * Conflict might be that a huge page has just been allocated
-		 * and added to page cache by a racing thread, or that there
-		 * is already at least one small page in the huge extent.
-		 * Be careful to retry when appropriate, but not forever!
-		 * Elsewhere -EEXIST would be the right code, but not here.
-		 */
-		if (xa_find(&mapping->i_pages, &index,
-				index + HPAGE_PMD_NR - 1, XA_PRESENT))
-			return ERR_PTR(-E2BIG);
+			/*
+			 * Check for conflict before waiting on a huge allocation.
+			 * Conflict might be that a huge page has just been allocated
+			 * and added to page cache by a racing thread, or that there
+			 * is already at least one small page in the huge extent.
+			 * Be careful to retry when appropriate, but not forever!
+			 * Elsewhere -EEXIST would be the right code, but not here.
+			 */
+			if (xa_find(&mapping->i_pages, &index,
+				    index + HPAGE_PMD_NR - 1, XA_PRESENT))
+				return ERR_PTR(-E2BIG);
+		}
 
-		folio = shmem_alloc_folio(gfp, HPAGE_PMD_ORDER, info, index);
-		if (!folio && pages == HPAGE_PMD_NR)
-			count_vm_event(THP_FILE_FALLBACK);
+		order = highest_order(suitable_orders);
+		while (suitable_orders) {
+			pages = 1UL << order;
+			index = round_down(index, pages);
+			folio = shmem_alloc_folio(gfp, order, info, index);
+			if (folio)
+				goto allocated;
+
+			if (pages == HPAGE_PMD_NR)
+				count_vm_event(THP_FILE_FALLBACK);
+			order = next_order(&suitable_orders, order);
+		}
  	} else {
  		pages = 1;
  		folio = shmem_alloc_folio(gfp, 0, info, index);

@@ -1664,6 +1782,7 @@ static struct folio *shmem_alloc_and_add_folio(gfp_t gfp,
  	if (!folio)
  		return ERR_PTR(-ENOMEM);
 
+allocated:
  	__folio_set_locked(folio);
  	__folio_set_swapbacked(folio);
 

@@ -1958,7 +2077,8 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
  	struct mm_struct *fault_mm;
  	struct folio *folio;
  	int error;
-	bool alloced;
+	bool alloced, huge;
+	unsigned long orders = 0;
 
  	if (WARN_ON_ONCE(!shmem_mapping(inode->i_mapping)))
  		return -EINVAL;

@@ -2030,14 +2150,21 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
  		return 0;
  	}
 

mm/shmem.c static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, · line 2156

Quoted 6 lines

mm/shmem.c +4 -2

-	if (shmem_is_huge(inode, index, false, fault_mm,
-			  vma ? vma->vm_flags : 0)) {
+	huge = shmem_is_huge(inode, index, false, fault_mm,
+			     vma ? vma->vm_flags : 0);
+	/* Find hugepage orders that are allowed for anonymous shmem. */
+	if (vma && vma_is_anon_shmem(vma))

I guess we do not want to check the anon path here either (in case you agree to merge this with tmpfs path).

mm/shmem.c static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, · line 2159

Quoted 4 lines

mm/shmem.c +3 -0

+		orders = anon_shmem_allowable_huge_orders(inode, vma, index, huge);
+	else if (huge)
+		orders = BIT(HPAGE_PMD_ORDER);

Why not handling this case inside allowable_huge_orders()?

mm/shmem.c static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, · line 2161

Quoted 3 lines

mm/shmem.c +2 -0

+
+	if (orders > 0) {

Does it make sense to handle these case anymore? Before, we had the huge path and order-0. If we handle all cases in allowable_orders() perhaps we can simplify this.

mm/shmem.c static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, · line 2164

Quoted 3 lines

mm/shmem.c +0 -0

  		gfp_t huge_gfp;
 
  		huge_gfp = vma_thp_gfp_mask(vma);

We are also setting this flag regardless of the final order. Meaning that suitable_orders() might return order-0 and yet we keep the huge gfp flag. Is that right?

Quoted 29 lines

mm/shmem.c +4 -4

  		huge_gfp = limit_gfp_mask(huge_gfp, gfp);
-		folio = shmem_alloc_and_add_folio(huge_gfp,
-				inode, index, fault_mm, true);
+		folio = shmem_alloc_and_add_folio(vmf, huge_gfp,
+				inode, index, fault_mm, orders);
  		if (!IS_ERR(folio)) {
  			if (folio_test_pmd_mappable(folio))
  				count_vm_event(THP_FILE_ALLOC);

@@ -2047,7 +2174,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
  			goto repeat;
  	}
 
-	folio = shmem_alloc_and_add_folio(gfp, inode, index, fault_mm, false);
+	folio = shmem_alloc_and_add_folio(vmf, gfp, inode, index, fault_mm, 0);
  	if (IS_ERR(folio)) {
  		error = PTR_ERR(folio);
  		if (error == -EEXIST)

@@ -2058,7 +2185,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 
  alloced:
  	alloced = true;
-	if (folio_test_pmd_mappable(folio) &&
+	if (folio_test_large(folio) &&
  	    DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) <
  					folio_next_index(folio) - 1) {
  		struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);

Signature · 1 line

2.39.3

base: mm-unstable
user-time sys_time faults_per_sec_per_cpu faults_per_sec
0.04s 3.10s 83516.416 2669684.890

...

mm-unstable + patchset, anon shmem 64K mTHP enabled
user-time sys_time faults_per_sec_per_cpu faults_per_sec
0.08s 0.31s 678630.231 17082522.495

Anonymous pages have already been supported for multi-size (mTHP) allocation through commit 19eaf44954df, that can allow THP to be configured through the sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.

However, the anonymous shmem will ignore the anonymous mTHP rule configured through the sysfs interface, and can only use the PMD-mapped THP, that is not reasonable. Many implement anonymous page sharing through mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, therefore, users expect to apply an unified mTHP strategy for anonymous pages, also including the anonymous shared pages, in order to enjoy the benefits of mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc.

As discussed in the bi-weekly MM meeting[1], the mTHP controls should control all of shmem, not only anonymous shmem, but support will be added iteratively. Therefore, this patch set starts with support for anonymous shmem.

The primary strategy is similar to supporting anonymous mTHP. Introduce a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled', which can have almost the same values as the top-level '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new additional "inherit" option and dropping the testing options 'force' and 'deny'. By default all sizes will be set to "never" except PMD size, which is set to "inherit". This ensures backward compatibility with the anonymous shmem enabled of the top level, meanwhile also allows independent control of anonymous shmem enabled for each mTHP.

Use the page fault latency tool to measure the performance of 1G anonymous shmem

with 32 threads on my machine environment with: ARM64 Architecture, 32 cores, 125G memory:
base: mm-unstable
user-time sys_time faults_per_sec_per_cpu faults_per_sec
0.04s 3.10s 83516.416 2669684.890

mm-unstable + patchset, anon shmem mTHP disabled
user-time sys_time faults_per_sec_per_cpu faults_per_sec
0.02s 3.14s 82936.359 2630746.027

mm-unstable + patchset, anon shmem 64K mTHP enabled
user-time sys_time faults_per_sec_per_cpu faults_per_sec
0.08s 0.31s 678630.231 17082522.495

From the data above, it is observed that the patchset has a minimal impact when mTHP is not enabled (some fluctuations observed during testing). When enabling 64K mTHP, there is a significant improvement of the page fault latency.

[1] https://lore.kernel.org/all/[email protected]/

Changes from v3:
- Drop 'force' and 'deny' testing options for each mTHP.
- Use new helper update_mmu_tlb_range(), per Lance.
- Update documentation to drop "anonymous thp" terminology, per David.
- Initialize the 'suitable_orders' in shmem_alloc_and_add_folio(), reported by kernel test robot.
- Fix the highest mTHP order in shmem_get_unmapped_area().
- Update some commit message.

Changes from v2:
- Rebased to mm/mm-unstable.
- Remove 'huge' parameter for shmem_alloc_and_add_folio(), per Lance.

Changes from v1:
- Drop the patch that re-arranges the position of highest_order() and next_order(), per Ryan.
- Modify the finish_fault() to fix VA alignment issue, per Ryan and David.
- Fix some building issues, reported by Lance and kernel test robot.
- Update some commit message.

Changes from RFC:
- Rebase the patch set against the new mm-unstable branch, per Lance.
- Add a new patch to export highest_order() and next_order().
- Add a new patch to align mTHP size in shmem_get_unmapped_area().
- Handle the uffd case and the VMA limits case when building mapping for large folio in the finish_fault() function, per Ryan.
- Remove unnecessary 'order' variable in patch 3, per Kefeng.
- Keep the anon shmem counters' name consistency.
- Modify the strategy to support mTHP for anonymous shmem, discussed with Ryan and David.
- Add reviewed tag from Barry.
- Update the commit message.

Baolin Wang (6):
mm: memory: extend finish_fault() to support large folio
mm: shmem: add THP validation for PMD-mapped THP related statistics
mm: shmem: add multi-size THP sysfs interface for anonymous shmem
mm: shmem: add mTHP support for anonymous shmem
mm: shmem: add mTHP size alignment in shmem_get_unmapped_area
mm: shmem: add mTHP counters for anonymous shmem

 Documentation/admin-guide/mm/transhuge.rst |  23 ++
 include/linux/huge_mm.h                    |  23 ++
 mm/huge_memory.c                           |  17 +-
 mm/memory.c                                |  57 +++-
 mm/shmem.c                                 | 344 ++++++++++++++++++---
 5 files changed, 403 insertions(+), 61 deletions(-)

Signature · 1 line

2.39.3

Hi Baolin,

Quoted 27 lines · Baolin Wang

Anonymous pages have already been supported for multi-size (mTHP) allocation through commit 19eaf44954df, that can allow THP to be configured through the sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.

However, the anonymous shmem will ignore the anonymous mTHP rule configured through the sysfs interface, and can only use the PMD-mapped THP, that is not reasonable. Many implement anonymous page sharing through mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, therefore, users expect to apply an unified mTHP strategy for anonymous pages, also including the anonymous shared pages, in order to enjoy the benefits of mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc.

As discussed in the bi-weekly MM meeting[1], the mTHP controls should control all of shmem, not only anonymous shmem, but support will be added iteratively. Therefore, this patch set starts with support for anonymous shmem.

The primary strategy is similar to supporting anonymous mTHP. Introduce a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled', which can have almost the same values as the top-level '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new additional "inherit" option and dropping the testing options 'force' and 'deny'. By default all sizes will be set to "never" except PMD size, which is set to "inherit". This ensures backward compatibility with the anonymous shmem enabled of the top level, meanwhile also allows independent control of anonymous shmem enabled for each mTHP.

Quoted 1 line

Use the page fault latency tool to measure the performance of 1G anonymous shmem

I'm not familiar with this tool. Could you share which repo/tool you are referring to?

Also, are you running or are you aware of any other tools/tests available for shmem that we can use to make sure we do not introduce any regressions?