[PATCH v4] mm/huge_memory: prevent losing PG_waiters in __split_folio_to_order()

[Zi Yan <ziy@nvidia.com>]

__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 setting PG_head along with other flags in new_folio->flags before
smp_wmb() and clear_compound_head(). The remaining compound page metadata
is set after clear_compound_head(). In that way, after
clear_compound_head() makes new_folio visible, nothing overwrites the
PG_waiters from softleaf_entry_wait_on_locked() by modifying
new_folio->flags non-atomically.

While at it, use data_race() to suppress the KCSAN issue reported by Ye
Liu's patch (the second Closes) on reading folio->flags in
__split_folio_to_order().

Suggested-by: Kiryl Shutsemau <kas@kernel.org>
Reported-by: David Hildenbrand <david@kernel.org>
Closes: https://lore.kernel.org/all/DMHE8BEEER1H.2BIY52NXATPIA@nvidia.com/
Reported-by: Ye Liu <ye.liu@linux.dev>
Closes: https://lore.kernel.org/all/20261029-data_race_huge_mem-v1-1-112e331291c1@linux.dev/
Fixes: c010d47f107f6 ("mm: thp: split huge page to any lower order pages")
Cc: stable@vger.kernel.org
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
The issue was raised by David when reviewing Ye Liu's "mm/huge_memory: use
READ_ONCE() when cloning folio flags during split" [1]. This patch also
supersedes Ye Liu's patch.

[1] https://lore.kernel.org/all/20261029-data_race_huge_mem-v1-1-112e331291c1@linux.dev/
---
Changes in v4:
- removed the data_race() on new_folio->flags read, since there is no data
  race on it.
- rephrased the comment above data_race() to avoid confusion.
- rephrased the comment on setting PG_head to clarify why it needs to be
  set before smp_wmb() and why no plain RMW is allowed after
  clear_compound_head().
- adjusted the comment on __prep_compound_page() about PG_head.
- Link to v3: https://patch.msgid.link/20261109-fix-split_folio_to_order-race-v3-1-1cdc509e34be@nvidia.com

Changes in v3:
- set PG_head before smp_wmb() like Kiryl suggested instead of
  prep_compound_page_atomic().
- used data_race() to suppress KCSAN reports while at it.
- dropped Lorenzo's ack, since the patch is changed.
- Link to v2: https://patch.msgid.link/20261106-fix-split_folio_to_order-race-v2-1-1b59c67b8af3@nvidia.com

Changes in v2:
- renamed __prep_compound_head_and_tail() to __prep_compound_page().
- added comments to prep_compound_page() and __prep_compound_page().
- Link to v1: https://patch.msgid.link/20261029-fix-split_folio_to_order-race-v1-1-a483c7451b0d@nvidia.com
---
 mm/huge_memory.c | 32 ++++++++++++++++++++++++++++----
 mm/internal.h    |  2 ++
 mm/page_alloc.c  | 17 +++++++++++++++--
 3 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bd44d58e86845..76c57d845269e 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;
+		memdesc_flags_t new_flags;
 
 		VM_BUG_ON_PAGE(atomic_read(&new_folio->_mapcount) != -1, new_head);
 
@@ -3730,8 +3731,15 @@ 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 &
+		new_flags.f = new_folio->flags.f & ~PAGE_FLAGS_CHECK_AT_PREP;
+		/*
+		 * data_race() is used because folio->flags is only read here,
+		 * but a reference-less writer can still write to it, e.g.,
+		 * PG_waiters update from softleaf_entry_wait_on_locked().
+		 * The flag is not transferred to new_flags as it is
+		 * folio specific, so it is not lost.
+		 */
+		new_flags.f |= data_race(folio->flags.f) &
 				((1L << PG_referenced) |
 				 (1L << PG_swapbacked) |
 				 (1L << PG_swapcache) |
@@ -3749,7 +3757,19 @@ 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 before smp_wmb() and clear_compound_head().
+		 * After clear_compound_head() marks new_head as a folio of its
+		 * own, new_head->flags must not be modified by plain RMWs.
+		 * A plain RMW can cause a loss of atomic PG_* updates from
+		 * a reference-less writer, e.g., setting PG_waiters via
+		 * softleaf_entry_wait_on_locked().
+		 */
+		if (new_order)
+			new_flags.f |= 1L << PG_head;
+		WRITE_ONCE(new_folio->flags.f, new_flags.f);
 
 		new_folio->mapping = folio->mapping;
 		new_folio->index = folio->index + i;
@@ -3779,7 +3799,11 @@ 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);
+			/*
+			 * PG_head is set above, so only set up the remaining
+			 * metadata on head page and tail pages.
+			 */
+			__prep_compound_page(new_head, new_order);
 			folio_set_large_rmappable(new_folio);
 		}
 
diff --git a/mm/internal.h b/mm/internal.h
index aeebb3e49ed11..4ce67ce2f3fba 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 143776e671abb..5f03f3725dd2e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -682,18 +682,31 @@ static inline bool pcp_allowed_order(unsigned int order)
  * This usage means that zero-order pages may not be compound.
  */
 
-void prep_compound_page(struct page *page, unsigned int order)
+/*
+ * Set up the metadata on head page and tail pages.
+ * Caller must set PG_head before this.
+ */
+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);
 
 	prep_compound_head(page, order);
 }
 
+/*
+ * PG_head is set non-atomically. It should be used when no one else can modify
+ * @page's flags.
+ */
+void prep_compound_page(struct page *page, unsigned int order)
+{
+	__SetPageHead(page);
+	__prep_compound_page(page, order);
+}
+
 static inline void set_buddy_order(struct page *page, unsigned int order)
 {
 	set_page_private(page, order);

---
base-commit: d865f2677c44cb2171b833d6775cf692756fa8be
change-id: 20261029-fix-split_folio_to_order-race-087a2a9fbf7c

Best regards,
--  
Yan, Zi