The patch titled Subject: mm/mglru: fix ineffective protection calculation has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-mglru-fix-ineffective-protection-calculation.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Yu Zhao yuzhao@google.com Subject: mm/mglru: fix ineffective protection calculation Date: Fri, 12 Jul 2024 17:29:56 -0600

mem_cgroup_calculate_protection() is not stateless and should only be used as part of a top-down tree traversal. shrink_one() traverses the per-node memcg LRU instead of the root_mem_cgroup tree, and therefore it should not call mem_cgroup_calculate_protection().

The existing misuse in shrink_one() can cause ineffective protection of sub-trees that are grandchildren of root_mem_cgroup. Fix it by reusing lru_gen_age_node(), which already traverses the root_mem_cgroup tree, to calculate the protection.

Previously lru_gen_age_node() opportunistically skips the first pass, i.e., when scan_control->priority is DEF_PRIORITY. On the second pass, lruvec_is_sizable() uses appropriate scan_control->priority, set by set_initial_priority() from lru_gen_shrink_node(), to decide whether a memcg is too small to reclaim from.

Now lru_gen_age_node() unconditionally traverses the root_mem_cgroup tree. So it should call set_initial_priority() upfront, to make sure lruvec_is_sizable() uses appropriate scan_control->priority on the first pass. Otherwise, lruvec_is_reclaimable() can return false negatives and result in premature OOM kills when min_ttl_ms is used.

Link: https://lkml.kernel.org/r/20240712232956.1427127-1-yuzhao@google.com Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists") Signed-off-by: Yu Zhao yuzhao@google.com Reported-by: T.J. Mercier tjmercier@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/vmscan.c | 30 ++++++++++++------------------ 1 file changed, 12 insertions(+), 18 deletions(-)

--- a/mm/vmscan.c~mm-mglru-fix-ineffective-protection-calculation +++ a/mm/vmscan.c @@ -3933,19 +3933,17 @@ static bool lruvec_is_reclaimable(struct struct mem_cgroup *memcg = lruvec_memcg(lruvec); DEFINE_MIN_SEQ(lruvec);

- /* see the comment on lru_gen_folio */ - gen = lru_gen_from_seq(min_seq[LRU_GEN_FILE]); - birth = READ_ONCE(lruvec->lrugen.timestamps[gen]); - - if (time_is_after_jiffies(birth + min_ttl)) + if (mem_cgroup_below_min(NULL, memcg)) return false;

if (!lruvec_is_sizable(lruvec, sc)) return false;

- mem_cgroup_calculate_protection(NULL, memcg); + /* see the comment on lru_gen_folio */ + gen = lru_gen_from_seq(min_seq[LRU_GEN_FILE]); + birth = READ_ONCE(lruvec->lrugen.timestamps[gen]);

- return !mem_cgroup_below_min(NULL, memcg); + return time_is_before_jiffies(birth + min_ttl); }

/* to protect the working set of the last N jiffies */ @@ -3955,23 +3953,20 @@ static void lru_gen_age_node(struct pgli { struct mem_cgroup *memcg; unsigned long min_ttl = READ_ONCE(lru_gen_min_ttl); + bool reclaimable = !min_ttl;

VM_WARN_ON_ONCE(!current_is_kswapd());

- /* check the order to exclude compaction-induced reclaim */ - if (!min_ttl || sc->order || sc->priority == DEF_PRIORITY) - return; + set_initial_priority(pgdat, sc);

memcg = mem_cgroup_iter(NULL, NULL, NULL); do { struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);

- if (lruvec_is_reclaimable(lruvec, sc, min_ttl)) { - mem_cgroup_iter_break(NULL, memcg); - return; - } + mem_cgroup_calculate_protection(NULL, memcg);

- cond_resched(); + if (!reclaimable) + reclaimable = lruvec_is_reclaimable(lruvec, sc, min_ttl); } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)));

/* @@ -3979,7 +3974,7 @@ static void lru_gen_age_node(struct pgli * younger than min_ttl. However, another possibility is all memcgs are * either too small or below min. */ - if (mutex_trylock(&oom_lock)) { + if (!reclaimable && mutex_trylock(&oom_lock)) { struct oom_control oc = { .gfp_mask = sc->gfp_mask, }; @@ -4772,8 +4767,7 @@ static int shrink_one(struct lruvec *lru struct mem_cgroup *memcg = lruvec_memcg(lruvec); struct pglist_data *pgdat = lruvec_pgdat(lruvec);

- mem_cgroup_calculate_protection(NULL, memcg); - + /* lru_gen_age_node() called mem_cgroup_calculate_protection() */ if (mem_cgroup_below_min(NULL, memcg)) return MEMCG_LRU_YOUNG;

_

Patches currently in -mm which might be from yuzhao@google.com are

mm-mglru-fix-ineffective-protection-calculation.patch