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

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

[Wei Yang]

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.

Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
Cc: <stable@vger.kernel.org>

---
This patch is based on current mm-new, latest commit:

    febb34c02328 dt-bindings: riscv: Add Svrsw60t59b extension description

v2:
  * just move folio->mapping ahead
---
 mm/huge_memory.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index efea42d68157..4e9e920f306d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3929,6 +3929,16 @@ 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 >= old_order)
 		return -EINVAL;
 
@@ -3965,18 +3975,6 @@ 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;
-- 
2.34.1

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

[Wei Yang]

On Wed, Nov 19, 2025 at 11:53:02PM +0000, Wei Yang wrote:
>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.
>
>Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
>Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>Cc: Zi Yan <ziy@nvidia.com>
>Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
>Cc: <stable@vger.kernel.org>
>
>---
>This patch is based on current mm-new, latest commit:
>
>    febb34c02328 dt-bindings: riscv: Add Svrsw60t59b extension description
>
>v2:
>  * just move folio->mapping ahead
>---
> mm/huge_memory.c | 22 ++++++++++------------
> 1 file changed, 10 insertions(+), 12 deletions(-)
>
>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>index efea42d68157..4e9e920f306d 100644
>--- a/mm/huge_memory.c
>+++ b/mm/huge_memory.c
>@@ -3929,6 +3929,16 @@ 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;
>+

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.

> 	if (new_order >= old_order)
> 		return -EINVAL;
> 
>@@ -3965,18 +3975,6 @@ 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;
>-- 
>2.34.1

-- 
Wei Yang
Help you, Help me

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

[Andrew Morton]

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.



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;
_

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

[Wei Yang]

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
Help you, Help me

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

[Zi Yan]

On 19 Nov 2025, at 18:53, Wei Yang wrote:

> 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.
>
> Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
> Cc: <stable@vger.kernel.org>
>
> ---
> This patch is based on current mm-new, latest commit:
>
>     febb34c02328 dt-bindings: riscv: Add Svrsw60t59b extension description
>
> v2:
>   * just move folio->mapping ahead
> ---
>  mm/huge_memory.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
>

Thanks. Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi

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

[Baolin Wang]

On 2025/11/20 07:53, Wei Yang wrote:
> 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.
> 
> Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
> Cc: <stable@vger.kernel.org>
> 
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

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

[David Hildenbrand (Red Hat)]

On 11/20/25 00:53, Wei Yang wrote:
> 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.
> 
> Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
> Cc: <stable@vger.kernel.org>

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

-- 
Cheers

David