Re: [PATCH v2] mm/huge_memory: prevent losing PG_waiters in __split_folio_to_order()

[Kiryl Shutsemau]

On Fri, Nov 06, 2026 at 01:28:46PM -0500, Zi Yan wrote:
> __split_folio_to_order() calls prep_compound_page() on each new head page
> to create a compound page after clear_compound_head(). prep_compound_page()
> uses non atomic __SetPageHead(). It is mostly OK, since the folio is locked
> and frozen and no one else is supposed to modify the folio's flags.
> 
> However, code calling migration_entry_wait() can reach the folio by PFN,
> then access its flags. It is because softleaf_entry_wait_on_locked() used
> by migration_entry_wait() sets PG_waiters concurrently without taking a
> reference and the frozen refcount during split cannot block it. Meanwhile,
> __SetPageHead() does a plain read and write, so PG_waiters atomically set
> by softleaf_entry_wait_on_locked() will be lost if the set happens after
> __SetPageHead()'s read and before its write.
> 
> One example code interleaving:
> 
> CPU0: __split_folio_to_order(new_order>0)  CPU1: do_swap_page()
> 
> 1. clear_compound_head(new_head)
>    new_head->compound_info = 0
>    (page_folio(new_head) == new_head now)
> 
>                                            2. migration_entry_wait()
>                                              softleaf_to_folio(entry)
>                                              page_folio() -> new_head
>                                              (not the old head any more)
> 
> 3. prep_compound_page(new_head, new_order)
>      __SetPageHead(new_head)
>        tmp = new_head->flags.f  <-- load; PG_waiters == 0 in tmp
> 
>                                            4. folio_set_waiters(new_head)
>                                              set_bit(PG_waiters, &flags.f)
>                                              enqueue on waitqueue, sleep
> 
> 5.     new_head->flags.f = tmp | (1UL << PG_head)
>        <-- store; overwrites step 4, PG_waiters == 0 again
> 
> Later, folio_unlock(new_folio) sees no PG_waiters, skips folio_wake_bit(),
> and CPU1 can remain blocked indefinitely.
> 
> Fix it by adding the atomic version SetPageHead() and
> prep_compound_page_atomic() using it. __split_folio_to_order() uses
> prep_compound_page_atomic() to avoid the issue without adding an atomic ops
> to other prep_compound_page() users.
> 
> Reported-by: David Hildenbrand <david@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/DMHE8BEEER1H.2BIY52NXATPIA@xxxxxxxxxx/
> Fixes: c010d47f107f6 ("mm: thp: split huge page to any lower order pages")
> Cc: stable@xxxxxxxxxxxxxxx
> Acked-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
> The issue was raised by David when reviewing Ye Liu's "mm/huge_memory: use
> READ_ONCE() when cloning folio flags during split" [1].
> 
> [1] https://lore.kernel.org/all/20261029-data_race_huge_mem-v1-1-112e331291c1@xxxxxxxxx/

Sorry for late reply, I only now got to this.

Setting PG_head atomically on its own looks odd to me. The comment above
smp_wmb() already says page flags must be visible before the page becomes
non-compound. prep_compound_page() writing PG_head after
clear_compound_head() is what breaks that.

Can we build the whole flags word for the new head, PG_head included, and
store it once before smp_wmb()?

After clear_compound_head() only the tail and head setup is left, and it
does not touch new_head->flags. Then SetPageHead() and
prep_compound_page_atomic() are not needed.

This was option 2 in your analysis[*] on Ye Liu's READ_ONCE() thread,
but I don't see a barrier problem.

PG_head gets the same ordering as the cloned PG_locked, and
softleaf_entry_wait_on_locked() already relies on that through the same
smp_wmb() and page_folio(). The new head carries PG_head for a moment
while it is still a tail, but page_folio() only looks at compound_info.

Reading the old head's flags with READ_ONCE() while building the word
would also cover Ye Liu's KCSAN report.

Something like this, on top of the v2 base, compile-tested only:

[*] https://lore.kernel.org/all/DMHE8BEEER1H.2BIY52NXATPIA@xxxxxxxxxx

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bd44d58e8684..53786b2abe89 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3714,6 +3714,7 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 		 * Don't pass it around before clear_compound_head().
 		 */
 		struct folio *new_folio = (struct folio *)new_head;
+		unsigned long flags;
 
 		VM_BUG_ON_PAGE(atomic_read(&new_folio->_mapcount) != -1, new_head);
 
@@ -3730,8 +3731,8 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 		 * unreferenced sub-pages of an anonymous THP: we can simply drop
 		 * PG_anon_exclusive (-> PG_mappedtodisk) for these here.
 		 */
-		new_folio->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
-		new_folio->flags.f |= (folio->flags.f &
+		flags = READ_ONCE(new_folio->flags.f) & ~PAGE_FLAGS_CHECK_AT_PREP;
+		flags |= READ_ONCE(folio->flags.f) &
 				((1L << PG_referenced) |
 				 (1L << PG_swapbacked) |
 				 (1L << PG_swapcache) |
@@ -3749,7 +3750,16 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 #endif
 				 (1L << PG_dirty) |
 				 (1L << PG_dropbehind) |
-				 LRU_GEN_MASK | LRU_REFS_MASK));
+				 LRU_GEN_MASK | LRU_REFS_MASK);
+
+		/*
+		 * Set PG_head here, not after clear_compound_head(): from then on
+		 * softleaf_entry_wait_on_locked() can set PG_waiters on new_head
+		 * without a reference, and a plain write would lose it.
+		 */
+		if (new_order)
+			flags |= 1L << PG_head;
+		WRITE_ONCE(new_folio->flags.f, flags);
 
 		new_folio->mapping = folio->mapping;
 		new_folio->index = folio->index + i;
@@ -3779,7 +3789,7 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 		 */
 		clear_compound_head(new_head);
 		if (new_order) {
-			prep_compound_page(new_head, new_order);
+			__prep_compound_page(new_head, new_order);
 			folio_set_large_rmappable(new_folio);
 		}
 
diff --git a/mm/internal.h b/mm/internal.h
index aeebb3e49ed1..4ce67ce2f3fb 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -807,6 +807,8 @@ static inline void prep_compound_tail(struct page *tail,
 	VM_WARN_ON_ONCE(tail->private);
 }
 
+void __prep_compound_page(struct page *page, unsigned int order);
+
 #if defined CONFIG_COMPACTION || defined CONFIG_CMA
 
 /*
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 143776e671ab..37768fa5341b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -683,11 +683,17 @@ static inline bool pcp_allowed_order(unsigned int order)
  */
 
 void prep_compound_page(struct page *page, unsigned int order)
+{
+	__SetPageHead(page);
+	__prep_compound_page(page, order);
+}
+
+/* Set up the head and tail pages of a page that already has PG_head */
+void __prep_compound_page(struct page *page, unsigned int order)
 {
 	int i;
 	int nr_pages = 1 << order;
 
-	__SetPageHead(page);
 	for (i = 1; i < nr_pages; i++)
 		prep_compound_tail(page + i, page, order);
 
-- 
  Kiryl Shutsemau / Kirill A. Shutemov