[PATCH v2] mm/huge_memory: Initialise workingset state before folio split

[Matt Fleming <matt@readmodwrite.com>]

From: Matt Fleming <mfleming@cloudflare.com>

xas_try_split() adds __GFP_ACCOUNT for page-cache xa_nodes, but
__folio_split() leaves the xa_state's xa_lru unset. That lets a live,
memcg-charged xa_node exist without being linked into the mapping's
shadow_nodes list_lru; when reclaim later walks the list_lru it trips
VM_WARN_ON(!css_is_dying()).

Use mapping_set_update() to install both the workingset update callback
and the shadow_nodes list_lru on the xa_state.

Reported-by: syzbot+c5b060ce82921a2fd500@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c5b060ce82921a2fd500
Fixes: 58729c04cf10 ("mm/huge_memory: add buddy allocator like (non-uniform) folio_split()")
Cc: stable@vger.kernel.org
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
---
Changes in v2:
- Move mapping_set_update() after filemap_release_folio() succeeds.
- Add Zi Yan's Reviewed-by.

Link: https://lore.kernel.org/linux-mm/20260724195244.3715130-1-matt@readmodwrite.com/
---
 mm/huge_memory.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b5d1e9d4463d..4ddbc72e92fd 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4033,7 +4033,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		gfp_t gfp;
 
 		mapping = folio->mapping;
-		min_order = mapping_min_folio_order(folio->mapping);
+		min_order = mapping_min_folio_order(mapping);
 		if (new_order < min_order) {
 			ret = -EINVAL;
 			goto out;
@@ -4047,6 +4047,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 			goto out;
 		}
 
+		mapping_set_update(&xas, mapping);
+
 		if (split_type == SPLIT_TYPE_UNIFORM) {
 			xas_set_order(&xas, folio->index, new_order);
 			xas_split_alloc(&xas, folio, old_order, gfp);
-- 
2.43.0