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

[Matt Fleming]

From: Matt Fleming <mfleming@xxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=c5b060ce82921a2fd500
Fixes: 58729c04cf10 ("mm/huge_memory: add buddy allocator like (non-uniform) folio_split()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Matt Fleming <mfleming@xxxxxxxxxxxxxx>
---
 mm/huge_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b5d1e9d4463d..12d0e0b14e83 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4033,7 +4033,8 @@ 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);
+		mapping_set_update(&xas, mapping);
+		min_order = mapping_min_folio_order(mapping);
 		if (new_order < min_order) {
 			ret = -EINVAL;
 			goto out;
-- 
2.43.0