Re: [Patch v2] mm/huge_memory: fix NULL pointer deference when splitting folio

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Nov 19, 2025 at 04:46:50PM -0800, Andrew Morton wrote:
>On Thu, 20 Nov 2025 00:03:12 +0000 Wei Yang <richard.weiyang@gmail.com> wrote:
>
>> +	 * TODO: this will also currently refuse shmem folios that are in the
>> >+	 * swapcache.
>> >+	 */
>> >+	if (!is_anon && !folio->mapping)
>> >+		return -EBUSY;
>> >+
>> 
>> This one would have a conflict on direct cherry-pick to current master and
>> mm-stable.
>> 
>> But if I move this code before (folio != page_folio(split_at) ...), it could
>> be apply to mm-new and master/mm-stable smoothly.
>> 
>> Not sure whether this could make Andrew's life easier.
>
>I added the below and fixed up fallout in the later patches.
>
>If this doesn't apply to -stable kernels then the -stable maintainers
>might later ask you to help rework it.
>

OK, got it.

>
>
>From: Wei Yang <richard.weiyang@gmail.com>
>Subject: mm/huge_memory: fix NULL pointer deference when splitting folio
>Date: Wed, 19 Nov 2025 23:53:02 +0000
>
>Commit c010d47f107f ("mm: thp: split huge page to any lower order pages")
>introduced an early check on the folio's order via mapping->flags before
>proceeding with the split work.
>
>This check introduced a bug: for shmem folios in the swap cache and
>truncated folios, the mapping pointer can be NULL.  Accessing
>mapping->flags in this state leads directly to a NULL pointer dereference.
>
>This commit fixes the issue by moving the check for mapping != NULL before
>any attempt to access mapping->flags.
>
>Link: https://lkml.kernel.org/r/20251119235302.24773-1-richard.weiyang@gmail.com
>Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
>Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>Reviewed-by: Zi Yan <ziy@nvidia.com>
>Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
>Cc: <stable@vger.kernel.org>
>Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>---
>
> mm/huge_memory.c |   22 ++++++++++------------
> 1 file changed, 10 insertions(+), 12 deletions(-)
>
>--- a/mm/huge_memory.c~mm-huge_memory-fix-null-pointer-deference-when-splitting-folio
>+++ a/mm/huge_memory.c
>@@ -3619,6 +3619,16 @@ static int __folio_split(struct folio *f
> 	if (folio != page_folio(split_at) || folio != page_folio(lock_at))
> 		return -EINVAL;
> 
>+	/*
>+	 * Folios that just got truncated cannot get split. Signal to the
>+	 * caller that there was a race.
>+	 *
>+	 * TODO: this will also currently refuse shmem folios that are in the
>+	 * swapcache.
>+	 */
>+	if (!is_anon && !folio->mapping)
>+		return -EBUSY;
>+
> 	if (new_order >= folio_order(folio))
> 		return -EINVAL;
> 
>@@ -3659,18 +3669,6 @@ static int __folio_split(struct folio *f
> 		gfp_t gfp;
> 
> 		mapping = folio->mapping;
>-
>-		/* Truncated ? */
>-		/*
>-		 * TODO: add support for large shmem folio in swap cache.
>-		 * When shmem is in swap cache, mapping is NULL and
>-		 * folio_test_swapcache() is true.
>-		 */
>-		if (!mapping) {
>-			ret = -EBUSY;
>-			goto out;
>-		}
>-
> 		min_order = mapping_min_folio_order(folio->mapping);
> 		if (new_order < min_order) {
> 			ret = -EINVAL;
>_

-- 
Wei Yang
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