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

[Andrew Morton]

On Thu, 16 Jul 2026 10:54:24 +0100 Kiryl Shutsemau <kirill@xxxxxxxxxxxxx> wrote:

> 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

Added as a hotfix. thanks.

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

Sashiko might have found a pre-existing bug in there:

	https://sashiko.dev/#/patchset/20260716095424.471052-1-kirill@xxxxxxxxxxxxx

(why is mapping_set_update() a) a macro and b) undocumented?)