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Do not shatter hugezeropage on wp-fault

PATCH Applied Dev Jain 1 participant 3 messages Oct 8, 2024

Subsystems (2)
Memory management THP
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Dev Jain

author reviewer
#0 lore raw

v7 cover Do not shatter hugezeropage on wp-fault

It was observed at [1] and [2] that the current kernel behaviour of shattering a hugezeropage is inconsistent and suboptimal. For a VMA with a THP allowable order, when we write-fault on it, the kernel installs a PMD-mapped THP. On the other hand, if we first get a read fault, we get a PMD pointing to the hugezeropage; subsequent write will trigger a write-protection fault, shattering the hugezeropage into one writable page, and all the other PTEs write-protected. The conclusion being, as compared to the case of a single write-fault, applications have to suffer 512 extra page faults if they were to use the VMA as such, plus we get the overhead of khugepaged trying to replace that area with a THP anyway.

Instead, replace the hugezeropage with a THP on wp-fault.

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

The patchset has been rebased on the mm-unstable branch.

v6->v7: Align function parameters in the second line by two tabs

v5->v6:
- More goto ommissions, remove build warning for !CONFIG_NUMA

v4->v5:
- Directly return VM_FAULT_FALLBACK in case of !folio

v3->v4:
- Renames: pmd_thp_fault_alloc -> vma_alloc_anon_folio_pmd, map_pmd_thp -> map_anon_folio_pmd
- Instead of passing around, compute haddr at various places, similar with gfp flags
- Pass haddr to update_mmu_cache_pmd() instead of unaligned address
- Do not pass vmf to map_anon_folio_pmd
- Do declarations in reverse xmas tree order
- Drop a new line which was introduced accidentally
- Call __pmd_thp_fault_success_stats from map_anon_folio_pmd
- Correctly return NULL from vma_alloc_anon_folio_pmd
- Initialize pgtable to NULL in __do_huge_pmd_anonymous_page, to prevent freeing pgtable when not even allocated
- Drop if conditions from map_anon_folio_pmd, let the caller handle that

v2->v3:
- Drop foliop and order parameters, prefix the thp functions with pmd_
- First allocate THP, then pgtable, not vice-versa
- Move pgtable_trans_huge_deposit() from map_pmd_thp() to caller
- Drop exposing functions in include/linux/huge_mm.h
- Open code do_huge_zero_wp_pmd_locked()
- Release folio in case of pmd change after taking the lock, or check_stable_address_space() returning VM_FAULT_SIGBUS
- Drop uffd-wp preservation. Looking at page_table_check_pmd_flags(), preserving uffd-wp on a writable entry is invalid. Looking at mfill_atomic(), uffd_copy() is a null operation when pmd is marked uffd-wp.

v1->v2:
- Wrap do_huge_zero_wp_pmd_locked() around lock and unlock
- Call thp_fault_alloc() before do_huge_zero_wp_pmd_locked() to avoid
- calling sleeping function from spinlock context

Dev Jain (2):
mm: Abstract THP allocation
mm: Allocate THP on hugezeropage wp-fault

 mm/huge_memory.c | 139 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 97 insertions(+), 42 deletions(-)
Signature · 1 line
2.30.2
author reviewer
#1 lore raw

PATCH v7 1/2 mm: Abstract THP allocation

In preparation for the second patch, abstract away the THP allocation logic present in the create_huge_pmd() path, which corresponds to the faulting case when no page is present.

There should be no functional change as a result of applying this patch, except that, as David notes at [1], a PMD-aligned address should be passed to update_mmu_cache_pmd().

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

Acked-by David Hildenbrand <[email protected]>
Reviewed-by Kefeng Wang <[email protected]>
Signed-off-by Dev Jain <[email protected]>
---
 mm/huge_memory.c | 98 ++++++++++++++++++++++++++++--------------------
 1 file changed, 57 insertions(+), 41 deletions(-)
mm/huge_memory.c +57 -41
index 3ca89e0279a7..e21c4f759e00 100644
@@ -1148,47 +1148,81 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
 }
 EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
 
-static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
-			struct page *page, gfp_t gfp)
+static struct folio *vma_alloc_anon_folio_pmd(struct vm_area_struct *vma,
+		unsigned long addr)
 {
-	struct vm_area_struct *vma = vmf->vma;
-	struct folio *folio = page_folio(page);
-	pgtable_t pgtable;
-	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
-	vm_fault_t ret = 0;
+	gfp_t gfp = vma_thp_gfp_mask(vma);
+	const int order = HPAGE_PMD_ORDER;
+	struct folio *folio;
 
-	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
+	folio = vma_alloc_folio(gfp, order, vma, addr & HPAGE_PMD_MASK, true);
 
+	if (unlikely(!folio)) {
+		count_vm_event(THP_FAULT_FALLBACK);
+		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
+		return NULL;
+	}
+
+	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
 	if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
 		folio_put(folio);
 		count_vm_event(THP_FAULT_FALLBACK);
 		count_vm_event(THP_FAULT_FALLBACK_CHARGE);
-		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
-		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
-		return VM_FAULT_FALLBACK;
+		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
+		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
+		return NULL;
 	}
 	folio_throttle_swaprate(folio, gfp);
 
-	pgtable = pte_alloc_one(vma->vm_mm);
-	if (unlikely(!pgtable)) {
-		ret = VM_FAULT_OOM;
-		goto release;
-	}
-
-	folio_zero_user(folio, vmf->address);
+	folio_zero_user(folio, addr);
 	/*
 	 * The memory barrier inside __folio_mark_uptodate makes sure that
 	 * folio_zero_user writes become visible before the set_pmd_at()
 	 * write.
 	 */
 	__folio_mark_uptodate(folio);
+	return folio;
+}
+
+static void map_anon_folio_pmd(struct folio *folio, pmd_t *pmd,
+		struct vm_area_struct *vma, unsigned long haddr)
+{
+	pmd_t entry;
+
+	entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
+	entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
+	folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
+	folio_add_lru_vma(folio, vma);
+	set_pmd_at(vma->vm_mm, haddr, pmd, entry);
+	update_mmu_cache_pmd(vma, haddr, pmd);
+	add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
+	count_vm_event(THP_FAULT_ALLOC);
+	count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
+	count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
+}
+
+static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
+{
+	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+	struct vm_area_struct *vma = vmf->vma;
+	struct folio *folio;
+	pgtable_t pgtable;
+	vm_fault_t ret = 0;
+
+	folio = vma_alloc_anon_folio_pmd(vma, vmf->address);
+	if (unlikely(!folio))
+		return VM_FAULT_FALLBACK;
+
+	pgtable = pte_alloc_one(vma->vm_mm);
+	if (unlikely(!pgtable)) {
+		ret = VM_FAULT_OOM;
+		goto release;
+	}
 
 	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
 	if (unlikely(!pmd_none(*vmf->pmd))) {
 		goto unlock_release;
 	} else {
-		pmd_t entry;
-
 		ret = check_stable_address_space(vma->vm_mm);
 		if (ret)
 			goto unlock_release;

@@ -1202,21 +1236,11 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
 			VM_BUG_ON(ret & VM_FAULT_FALLBACK);
 			return ret;
 		}
-
-		entry = mk_huge_pmd(page, vma->vm_page_prot);
-		entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
-		folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
-		folio_add_lru_vma(folio, vma);
 		pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
-		set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
-		update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
-		add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
+		map_anon_folio_pmd(folio, vmf->pmd, vma, haddr);
 		mm_inc_nr_ptes(vma->vm_mm);
 		deferred_split_folio(folio, false);
 		spin_unlock(vmf->ptl);
-		count_vm_event(THP_FAULT_ALLOC);
-		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
-		count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
 	}
 
 	return 0;

@@ -1283,8 +1307,6 @@ static void set_huge_zero_folio(pgtable_t pgtable, struct mm_struct *mm,
 vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
-	gfp_t gfp;
-	struct folio *folio;
 	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
 	vm_fault_t ret;
 

@@ -1335,14 +1357,8 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 		}
 		return ret;
 	}
-	gfp = vma_thp_gfp_mask(vma);
-	folio = vma_alloc_folio(gfp, HPAGE_PMD_ORDER, vma, haddr, true);
-	if (unlikely(!folio)) {
-		count_vm_event(THP_FAULT_FALLBACK);
-		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
-		return VM_FAULT_FALLBACK;
-	}
-	return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp);
+
+	return __do_huge_pmd_anonymous_page(vmf);
 }
 
 static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
Signature · 1 line
2.30.2
author reviewer
#2 lore raw

PATCH v7 2/2 mm: Allocate THP on hugezeropage wp-fault

In reply to Dev Jain

Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and replace it with a PMD-mapped THP. Remember to flush TLB entry corresponding to the hugezeropage. In case of failure, fallback to splitting the PMD.

Acked-by David Hildenbrand <[email protected]>
Reviewed-by Kefeng Wang <[email protected]>
Signed-off-by Dev Jain <[email protected]>
---
 mm/huge_memory.c | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)
mm/huge_memory.c +40 -1
index e21c4f759e00..606ed782a15a 100644
@@ -1790,6 +1790,38 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
 	spin_unlock(vmf->ptl);
 }
 
+static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf)
+{
+	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+	struct vm_area_struct *vma = vmf->vma;
+	struct mmu_notifier_range range;
+	struct folio *folio;
+	vm_fault_t ret = 0;
+
+	folio = vma_alloc_anon_folio_pmd(vma, vmf->address);
+	if (unlikely(!folio))
+		return VM_FAULT_FALLBACK;
+
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
+				haddr + HPAGE_PMD_SIZE);
+	mmu_notifier_invalidate_range_start(&range);
+	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
+	if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
+		goto release;
+	ret = check_stable_address_space(vma->vm_mm);
+	if (ret)
+		goto release;
+	(void)pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
+	map_anon_folio_pmd(folio, vmf->pmd, vma, haddr);
+	goto unlock;
+release:
+	folio_put(folio);
+unlock:
+	spin_unlock(vmf->ptl);
+	mmu_notifier_invalidate_range_end(&range);
+	return ret;
+}
+
 vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
 {
 	const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;

@@ -1802,8 +1834,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
 	vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
 	VM_BUG_ON_VMA(!vma->anon_vma, vma);
 
-	if (is_huge_zero_pmd(orig_pmd))
+	if (is_huge_zero_pmd(orig_pmd)) {
+		vm_fault_t ret = do_huge_zero_wp_pmd(vmf);
+
+		if (!(ret & VM_FAULT_FALLBACK))
+			return ret;
+
+		/* Fallback to splitting PMD if THP cannot be allocated */
 		goto fallback;
+	}
 
 	spin_lock(vmf->ptl);
Signature · 1 line
2.30.2