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

[Miaohe Lin]

On 2026/7/16 17:54, Kiryl Shutsemau 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
> 
>   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>

Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>

Thanks.
.