Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes

[Lance Yang <lance.yang@linux.dev>]

On Thu, Nov 05, 2026 at 11:06:29AM +0100, David Hildenbrand (Arm) wrote:
>On 11/5/26 10:00, Baolin Wang wrote:
>> CC Barry, who added the TTU_SYNC in shrink_folio_list().
>> 
>> On 11/5/26 1:48 PM, Lance Yang wrote:
>>>
>>>
>>> On 2026/11/5 12:54, Lance Yang wrote:
>>>>
>>>>
>>>> Good catch!
>>>>
>>>>
>>>> Right. Whether the folio can be PMD-mapped doesn't tell us whether its
>>>> PTE table can be reclaimed, so those extra checks can skip synchronization
>>>> that the caller explicitly asked for ;)
>>>
>>> Ouch ... one thing I missed ... what about order-0 reclaim?
>>>
>>> shrink_folio_list() only sets TTU_SYNC for large folio, so can't we still
>>> hit the same race there?
>> 
>> Thanks Lance. But this matches the purpose of the TTU_SYNC flag, which is used
>> to avoid racy checks with PVMW_SYNC. If no TTU_SYNC flag is set, it means the
>> caller can tolerate some racy checks.
>> 
>> For this case, I think we should set TTU_SYNC unconditionally in
>> shrink_folio_list(). Barry, what do you think?
>
>That makes sense. I hope this doesn't throw a big wrench into our reclaim
>performance numbers.
>
>Can someone benchmark this?
>
>But maybe we should really follow-up with the folio-size based optimization in
>sync_with_folio_pmd_zap(), where we only need a memory barrier (and sufficient
>documentation why something that cannot be PMD-mapped can get away cheaper).

Dug into the history a bit :D

TTU_SYNC goes back to Hugh's THP splitting fix [1]. A racing zap could
clear the PTE/PMD before updating mapcount, so the walker could skip the
lock and return before the accounting caught up.

Barry later enabled it for large-folio reclaim [2], to avoid skipping
temporarily cleared PTEs and ending up with a partially unmapped folio.
He mentioned that small folios had the same race, but with just one PTE,
they couldn't be partially unmapped. That's where the large-folio check
in shrink_folio_list() came from.

Then PTE table reclaim [3] added another way to end up with an empty PMD,
even for a folio that can't be PMD-mapped. The truncation fix [4] moved
the rmap flush before the PMD clear and extended synchronization in
zap_pmd_range() to cover those folios too.

Looking back at the truncation discussion [5], you also pointed out that
page_vma_mapped_walk() needed checking, and suggested a possible
folio-size based optimization in the helper.

[1] https://lore.kernel.org/all/c1e95853-8bcd-d8fd-55fa-e7f2488e78f@google.com/
[2] https://lore.kernel.org/all/20240306095219.71086-1-21cnbao@gmail.com/
[3] https://lore.kernel.org/all/92aba2b319a734913f18ba41e7d86a265f0b84e2.1733305182.git.zhengqi.arch@bytedance.com/
[4] https://lore.kernel.org/all/14ebbd556cc4941f0ebc43da59fa6a2fdca25dc8.1793148609.git.baolin.wang@linux.alibaba.com/
[5] https://lore.kernel.org/all/08a0907f-8e3a-4071-a2e4-fd1411c446f3@kernel.org/

So, how about starting with order-0 folios?

---8<---
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 74cca0e3c726..d01fe8282417 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -872,6 +872,8 @@ struct page_vma_mapped_walk {
 	spinlock_t *ptl;
 	unsigned int flags;
 	bool pgoff_is_anon : 1;
+	/* Set from the folio, not the number of pages being walked. */
+	bool folio_is_order0 : 1;
 };

 #define DEFINE_FOLIO_VMA_WALK(name, _folio, _vma, _address, _flags)	\
@@ -883,6 +885,7 @@ struct page_vma_mapped_walk {
 		.address = _address,					\
 		.flags = _flags,					\
 		.pgoff_is_anon = folio_test_anon(_folio),		\
+		.folio_is_order0 = !folio_test_large(_folio),	\
 	}

 static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
diff --git a/mm/internal.h b/mm/internal.h
index 9b8864a8540a..e70869b38292 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -592,21 +592,29 @@ void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte);
  * sync_with_folio_pmd_zap - sync with concurrent zapping of folios
  * @mm: The mm_struct.
  * @pmdp: Pointer to the pmd that was found to be pmd_none().
+ * @order0: True if the target is known to be an order-0 folio.
  *
- * When we find a pmd_none() while unmapping a folio without holding the PTL,
- * zap_huge_pmd() may have cleared the PMD but not yet updated the folio
- * mapcount, and zap_empty_pte_table() clears the PMD only after the rmaps
- * for the table have been removed, but nothing orders the pmd_none() read
- * against later mapcount reads. Skipping the PMD without synchronization
- * could make folio unmapping code assume that unmapping failed.
+ * An empty PMD does not guarantee that a later mapcount read sees the
+ * completed rmap updates. For PMD-mapped folios, zap_huge_pmd() clears
+ * the PMD before updating mapcount, so we must wait for the PMD lock.
  *
- * Wait for concurrent zapping to complete by grabbing the PTL.
+ * Order-0 folios cannot be PMD-mapped. Their fully ordered
+ * atomic_add_negative() updates mapcount before PTE table reclaim clears
+ * the PMD. The PTE lock preserves this ordering if another CPU reclaims
+ * the table. A read barrier therefore suffices for order-0 folios.
+ * Keep taking the PMD lock for all other folios.
  */
-static inline void sync_with_folio_pmd_zap(struct mm_struct *mm, pmd_t *pmdp)
+static inline void sync_with_folio_pmd_zap(struct mm_struct *mm, pmd_t *pmdp,
+					   bool order0)
 {
-	spinlock_t *ptl = pmd_lock(mm, pmdp);
+	if (order0) {
+		/* Order the PMD read before subsequent mapcount reads. */
+		smp_rmb();
+	} else {
+		spinlock_t *ptl = pmd_lock(mm, pmdp);

-	spin_unlock(ptl);
+		spin_unlock(ptl);
+	}
 }

 struct zap_details;
diff --git a/mm/memory.c b/mm/memory.c
index 44376321180b..8955d1cd31b0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2112,7 +2112,8 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
 			/* fall through */
 		} else if (pmd_none(*pmd)) {
 			if (details && details->single_folio)
-				sync_with_folio_pmd_zap(tlb->mm, pmd);
+				sync_with_folio_pmd_zap(tlb->mm, pmd,
+							!folio_test_large(details->single_folio));
 			addr = next;
 			continue;
 		}
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index e6b6170c2f4f..36881f8fd45e 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -293,11 +293,9 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 			spin_unlock(pvmw->ptl);
 			pvmw->ptl = NULL;
 		} else if (!pmd_present(pmde)) {
-			if ((pvmw->flags & PVMW_SYNC) &&
-			    thp_vma_suitable_order(vma, pvmw->address,
-						   PMD_ORDER) &&
-			    (pvmw->nr_pages >= HPAGE_PMD_NR))
-				sync_with_folio_pmd_zap(mm, pvmw->pmd);
+			if (pvmw->flags & PVMW_SYNC)
+				sync_with_folio_pmd_zap(mm, pvmw->pmd,
+							pvmw->folio_is_order0);

 			goto next_pmd;
 		}
@@ -379,6 +377,7 @@ unsigned long page_mapped_in_vma(const struct page *page,
 		.nr_pages = 1,
 		.vma = vma,
 		.flags = PVMW_SYNC,
+		.folio_is_order0 = !folio_test_large(folio),
 	};

 	if (folio_test_anon(folio))
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7db6be53f3a9..79cfa9bc475b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1434,26 +1434,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		 * processes. Try to unmap it here.
 		 */
 		if (folio_mapped(folio)) {
-			enum ttu_flags flags = TTU_BATCH_FLUSH;
+			enum ttu_flags flags = TTU_BATCH_FLUSH | TTU_SYNC;
 			bool was_swapbacked = folio_test_swapbacked(folio);

 			if (folio_test_pmd_mappable(folio))
 				flags |= TTU_SPLIT_HUGE_PMD;
+
 			/*
-			 * Without TTU_SYNC, try_to_unmap will only begin to
-			 * hold PTL from the first present PTE within a large
-			 * folio. Some initial PTEs might be skipped due to
-			 * races with parallel PTE writes in which PTEs can be
-			 * cleared temporarily before being written new present
-			 * values. This will lead to a large folio is still
-			 * mapped while some subpages have been partially
-			 * unmapped after try_to_unmap; TTU_SYNC helps
-			 * try_to_unmap acquire PTL from the first PTE,
-			 * eliminating the influence of temporary PTE values.
+			 * Synchronize with concurrent rmap updates before checking
+			 * folio_mapped(). Also avoid skipping temporarily cleared
+			 * PTEs, which could leave a large folio partially mapped.
 			 */
-			if (folio_test_large(folio))
-				flags |= TTU_SYNC;
-
 			try_to_unmap(folio, flags);
 			if (folio_mapped(folio)) {
 				stat->nr_unmap_fail += nr_pages;
--

atomic_add_negative() provides full ordering for mapcount updates. With
243cf1f67b36, rmap removal completes before clearing PMD, including when
another CPU reclaims an empty table under PTE lock. So smp_rmb() is enough
here, without an extra writer-side barrier.

nr_pages == 1 isn't enough to identify an order-0 folio, since
page_mapped_in_vma() also checks individual pages of large folios.

We can then enable TTU_SYNC for order-0 reclaim and use smp_rmb() on an
empty PMD, while keeping PMD locking for large folios. 

(Haven't benchmarked it yet) ...

Cheers, Lance