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

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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: 3200802728 ("mm/demotion: demote pages according to allocation fallback order")
Signed-off-by: Bing Jiao <bingjiao@google.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo 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: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Bing Jiao
2026-03-03 05:25:17 +00:00
committed by Andrew Morton
parent 909632714f
commit d9f74cfb5a
1 changed files with 5 additions and 9 deletions
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14
mm/vmscan.c
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@@ -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);
}