[PATCH v3] mm/huge_memory: unlock i_mmap_rwsem before releasing after-split folios

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

__folio_split() keeps dereferencing the mapping after the split:
shmem_uncharge(mapping->host) and remap_page() while the folios are still
frozen/locked, and i_mmap_unlock_read(mapping) at the very end, after the
after-split folios have been unlocked and freed.

Nothing holds an inode reference across that. The split relies on @folio
-- which the beyond-EOF drop loop never removes, as it starts at
folio_next(folio) -- staying locked and in the page cache to hold off
eviction. But the unlock loop unlocks @folio before i_mmap_unlock_read()
runs. If the caller's @lock_at is a tail beyond EOF, as memory_failure()
passes when splitting a poisoned tail of a shmem THP that reaches past
i_size during truncation, it too is gone from the page cache; so once
@folio is unlocked no locked, in-cache folio pins the inode, and a
concurrent final iput() can evict and RCU-free it before
i_mmap_unlock_read() touches i_mmap_rwsem:

  BUG: KASAN: slab-use-after-free in __up_read+0x634/0x790
   i_mmap_unlock_read include/linux/fs.h:537 [inline]
   __folio_split+0x732/0x1640 mm/huge_memory.c:4100
   try_to_split_thp_page+0xab/0x390 mm/memory-failure.c:1675
   memory_failure+0x1394/0x26e0 mm/memory-failure.c:2470

  Freed by task 4601:
   shmem_free_in_core_inode+0x54/0xb0 mm/shmem.c:5177
   evict+0x57f/0xac0 fs/inode.c:870

Do every mapping dereference while @folio still pins the inode: drop
i_mmap_rwsem right after remap_page(), before the loop that unlocks and
frees the after-split folios, and clear @mapping so the exit path does not
unlock it again. shmem_uncharge() and remap_page() already run before that
point, so after this nothing past the unlock loop touches the inode or the
mapping.

This is now a rule the split depends on, alongside keeping @folio frozen
until the page cache is updated: no inode or mapping dereference once the
after-split folios start being unlocked.

Reported-by: Hao Zhang <zhanghao1@xxxxxxxxxx>
Closes: https://lore.kernel.org/linux-mm/20260710071344.GA106129@zh-pc
Fixes: baa355fd3314 ("thp: file pages support for split_huge_page()")
Cc: <stable@xxxxxxxxxxxxxxx>
Co-developed-by: Hao Zhang <zhanghao1@xxxxxxxxxx>
Signed-off-by: Hao Zhang <zhanghao1@xxxxxxxxxx>
Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---

Same patch as posted in the v2 discussion [1], now with tags collected.

v3:
 - Move i_mmap_unlock_read() before the after-split unlock loop instead
   of pinning the inode or refusing the split; based on Hao's original
   patch [2] with the race analysis corrected.
v2 [3]:
 - Anchor the split on the head in memory_failure() (broken: the caller's
   reference must land on the poisoned page) + -EBUSY guard in
   __folio_split() (over-rejects for new_order > 0).
v1 [4]:
 - igrab()/iput() around the split (self-deadlocks on final iput() under
   the folio lock).

[1] https://lore.kernel.org/linux-mm/aldjhtfVByHDQXe6@thinkstation
[2] https://lore.kernel.org/linux-mm/20260710071344.GA106129@zh-pc
[3] https://lore.kernel.org/linux-mm/20260714122344.351895-1-kirill@xxxxxxxxxxxxx
[4] https://lore.kernel.org/linux-mm/20260713170915.239819-1-kirill@xxxxxxxxxxxxx

 mm/huge_memory.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2bccb0a53a0a..abaea34ef558 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4109,6 +4109,18 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 
 	remap_page(folio, 1 << old_order, ttu_flags);
 
+	/*
+	 * Drop the mapping while the inode is still pinned. @folio stays
+	 * locked and present in the page cache until the loop below, so
+	 * eviction cannot free the inode yet; @lock_at is not enough, it may
+	 * be a tail beyond EOF that the split already dropped from the page
+	 * cache. Nothing past this point may touch the inode or the mapping.
+	 */
+	if (mapping) {
+		i_mmap_unlock_read(mapping);
+		mapping = NULL;
+	}
+
 	/*
 	 * Unlock all after-split folios except the one containing
 	 * @lock_at page. If @folio is not split, it will be kept locked.

base-commit: 0e35b9b6ec0ffcc5e23cbdec09f5c622ad532b53
-- 
2.54.0