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

Lance Yang
2026-03-02T15:11:52Z

On 2026/3/2 22:28, David Hildenbrand (Arm) wrote:
> On 2/28/26 04:10, Lance Yang wrote:
>>
>>
>> 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@dfinity.org>
>>> Closes: https://lore.kernel.org/all/CAKNNEtw5_kZomhkugedKMPOG-
>>> sxs5Q5OLumWJdiWXv+C9Yct0w@mail.gmail.com/
>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>> Cc: <stable@vger.kernel.org>
>>> ---
>>
>> Thanks for the fix!
>>
>> I also made a C reproducer and tested this patch - the corruption
>> disappeared.
>
> Should we link that reproducer somehow from the patch description?

Yes, the original reproducer provided by Bas is available here[1].

Regarding the C reproducer, Zi plans to add it to selftests in a
follow-up patch (as we discussed off-list).

[1] https://github.com/dfinity/thp-madv-remove-test

Cheers,
Lance