Commit cff47b9e authored Nov 19, 2025 by Wei Yang Committed by Andrew Morton Nov 24, 2025

mm/huge_memory: fix NULL pointer deference when splitting folio

Commit c010d47f ("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: c010d47f ("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>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent 6c96c6bd

mm/huge_memory.c

+10 −12

Original line number	Diff line number	Diff line
		@@ -3619,6 +3619,16 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
		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 *folio, unsigned int new_order,
		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;