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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/2/26 21:31, 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 |
> +----------------+---------+----+----+
> 
> 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.
> 
> Fixes: 00527733d0dc8 ("mm/huge_memory: add two new (not yet used) functions for folio_split()")
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Reported-by: Bas van Dijk <bas@dfinity.org>
> Closes: https://lore.kernel.org/all/CAKNNEtw5_kZomhkugedKMPOG-sxs5Q5OLumWJdiWXv+C9Yct0w@mail.gmail.com/
> Tested-by: Lance Yang <lance.yang@linux.dev>
> Cc: <stable@vger.kernel.org>
> ---
>  mm/huge_memory.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 56db54fa48181..f0bdac3f270b5 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3647,6 +3647,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  	const bool is_anon = folio_test_anon(folio);
>  	int old_order = folio_order(folio);
>  	int start_order = split_type == SPLIT_TYPE_UNIFORM ? new_order : old_order - 1;
> +	struct folio *old_folio = folio;
>  	int split_order;
>  
>  	/*
> @@ -3668,11 +3669,17 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  			 * irq is disabled to allocate enough memory, whereas
>  			 * non-uniform split can handle ENOMEM.
>  			 */
> -			if (split_type == SPLIT_TYPE_UNIFORM)
> -				xas_split(xas, folio, old_order);
> -			else {

Just wondering whether we should no move the comment over here now, so
it just covers both cases.

> +			if (split_type == SPLIT_TYPE_UNIFORM) {
> +				xas_split(xas, old_folio, old_order);
> +			} else {
>  				xas_set_order(xas, folio->index, split_order);
> -				xas_try_split(xas, folio, old_order);
> +				/*
> +				 * use the to-be-split folio, so that a parallel
> +				 * folio_try_get() waits on it until xarray is
> +				 * updated with after-split folios and
> +				 * the original one is unfrozen.
> +				 */
> +				xas_try_split(xas, old_folio, old_order);
>  				if (xas_error(xas))
>  					return xas_error(xas);
>  			}

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David