[PATCH v2] mm/vmscan: fix unintended mtc->nmask mutation in alloc_demote_folio()

[Bing Jiao <bingjiao@google.com>]

In alloc_demote_folio(), mtc->nmask is set to NULL for the first
allocation. If that succeeds, it returns without restoring mtc->nmask
to allowed_mask. For subsequent allocations from the migrate_pages()
batch, mtc->nmask will be NULL. If the target node then becomes full,
the fallback allocation will use nmask = NULL, allocating from any
node allowed by the task cpuset, which for kswapd is all nodes.

To address this issue, use a local copy of the mtc structure with
nmask = NULL for the first allocation attempt specifically, ensuring
the original mtc remains unmodified.

Fixes: 320080272892 ("mm/demotion: demote pages according to allocation fallback order")
Signed-off-by: Bing Jiao <bingjiao@google.com>
---
 mm/vmscan.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cbffc0a27824..c4e0ce737e03 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -966,13 +966,11 @@ static void folio_check_dirty_writeback(struct folio *folio,
 static struct folio *alloc_demote_folio(struct folio *src,
 		unsigned long private)
 {
+	struct migration_target_control *mtc, target_nid_mtc;
 	struct folio *dst;
-	nodemask_t *allowed_mask;
-	struct migration_target_control *mtc;

 	mtc = (struct migration_target_control *)private;

-	allowed_mask = mtc->nmask;
 	/*
 	 * make sure we allocate from the target node first also trying to
 	 * demote or reclaim pages from the target node via kswapd if we are
@@ -982,15 +980,13 @@ static struct folio *alloc_demote_folio(struct folio *src,
 	 * a demotion of cold pages from the target memtier. This can result
 	 * in the kernel placing hot pages in slower(lower) memory tiers.
 	 */
-	mtc->nmask = NULL;
-	mtc->gfp_mask |= __GFP_THISNODE;
-	dst = alloc_migration_target(src, (unsigned long)mtc);
+	target_nid_mtc = *mtc;
+	target_nid_mtc.nmask = NULL;
+	target_nid_mtc.gfp_mask |= __GFP_THISNODE;
+	dst = alloc_migration_target(src, (unsigned long)&target_nid_mtc);
 	if (dst)
 		return dst;

-	mtc->gfp_mask &= ~__GFP_THISNODE;
-	mtc->nmask = allowed_mask;
-
 	return alloc_migration_target(src, (unsigned long)mtc);
 }

--
2.53.0.473.g4a7958ca14-goog