Re: [PATCH v2] mm/huge_memory: fix a folio_split() race condition with folio_try_get()

[Wei Yang]

On Mon, Mar 02, 2026 at 03:31:59PM -0500, 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. More detailed explanation is at the
>bottom.
>
>The reproducer is at: https://github.com/dfinity/thp-madv-remove-test
>It
>1. creates a memfd,
>2. forks,
>3. in the child process, maps the file with large folios (via shmem code
>   path) and reads the mapped file continuously with 16 threads,
>4. in the parent process, uses madvise(MADV_REMOVE) to punch poles in the
>   large folio.
>
>Data corruption can be observed without the fix. Basically, data from a
>wrong page->index is returned.
>
>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. Change xas_split() used in uniform split branch to use
>the original folio to avoid confusion.
>
>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").
>
>More details:
>
>For example, a folio f is split non-uniformly into f, f2, f3, f4 like
>below:
>+----------------+---------+----+----+
>|       f        |    f2   | f3 | f4 |
>+----------------+---------+----+----+
>but the xarray would look like below after __split_unmapped_folio() is
>done:
>+----------------+---------+----+----+
>|       f        |    f2   | f3 | f3 |
>+----------------+---------+----+----+
>

Thanks for the detailed explanation, I finally realized it behaves like this.

>After __split_unmapped_folio(), the code changes the xarray and unfreezes
>after-split folios:
>
>1. unfreezes f2, __xa_store(f2)
>2. unfreezes f3, __xa_store(f3)
>3. unfreezes f4, __xa_store(f4), which overwrites the second f3 to f4.
>4. unfreezes f.
>
>Meanwhile, a parallel filemap_get_entry() can read the second f3 from the
>xarray and use folio_try_get() on it at step 2 when f3 is unfrozen. Then,
>f3 is wrongly returned to user.
>
>After the fix, the xarray looks like below after __split_unmapped_folio():
>+----------------+---------+----+----+
>|       f        |    f    | f  | f  |
>+----------------+---------+----+----+
>so that the race window no longer exists.

Since we unfreeze f at last.

>
>Fixes: 00527733d0dc8 ("mm/huge_memory: add two new (not yet used) functions for folio_split()")
>Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>Reported-by: Bas van Dijk <bas@xxxxxxxxxxx>
>Closes: https://lore.kernel.org/all/CAKNNEtw5_kZomhkugedKMPOG-sxs5Q5OLumWJdiWXv+C9Yct0w@xxxxxxxxxxxxxx/
>Tested-by: Lance Yang <lance.yang@xxxxxxxxx>
>Cc: <stable@xxxxxxxxxxxxxxx>

So thanks for the fix.

Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxx>

-- 
Wei Yang
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