Commit d9f74cfb authored by Bing Jiao's avatar Bing Jiao Committed by Andrew Morton
Browse files

mm/vmscan: fix unintended mtc->nmask mutation in alloc_demote_folio() 

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.

Link: https://lkml.kernel.org/r/20260303052519.109244-1-bingjiao@google.com


Fixes: 32008027 ("mm/demotion: demote pages according to allocation fallback order")
Signed-off-by: default avatarBing Jiao <bingjiao@google.com>
Acked-by: default avatarDavid Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: default avatarLorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 90963271

mm/vmscan.c

+5 −9
Original line number Diff line number Diff line
@@ -985,13 +985,11 @@ 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
@@ -1001,15 +999,13 @@ 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);

}