Re: [PATCH] mm/huge_memory: fix a folio_split() race condition with folio_try_get()

[Lance Yang]

On 2026/2/28 09:06, Zi Yan wrote:
> During a pagecache folio split, the values in the related xarray should not
> be changed from the original folio at xarray split time until all
> after-split folios are well formed and stored in the xarray. Current use
> of xas_try_split() in __split_unmapped_folio() lets some after-split folios
> show up at wrong indices in the xarray. When these misplaced after-split
> folios are unfrozen, before correct folios are stored via __xa_store(), and
> grabbed by folio_try_get(), they are returned to userspace at wrong file
> indices, causing data corruption.
>
> Fix it by using the original folio in xas_try_split() calls, so that
> folio_try_get() can get the right after-split folios after the original
> folio is unfrozen.
>
> Uniform split, split_huge_page*(), is not affected, since it uses
> xas_split_alloc() and xas_split() only once and stores the original folio
> in the xarray.
>
> Fixes below points to the commit introduces the code, but folio_split() is
> used in a later commit 7460b470a131f ("mm/truncate: use folio_split() in
> truncate operation").
>
> Fixes: 00527733d0dc8 ("mm/huge_memory: add two new (not yet used) functions for folio_split()")
> Reported-by: Bas van Dijk <bas@xxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/CAKNNEtw5_kZomhkugedKMPOG-sxs5Q5OLumWJdiWXv+C9Yct0w@xxxxxxxxxxxxxx/
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---

Thanks for the fix!

I also made a C reproducer and tested this patch - the corruption
disappeared.

Without patch: corruption in < 10 iterations
With patch: 1000 iterations, all clean

Tested-by: Lance Yang <lance.yang@xxxxxxxxx>