+ mm-mglru-fix-ineffective-protection-calculation.patch added to mm-hotfixes-unstable branch

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

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@xxxxxxxxxx>
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@xxxxxxxxxx
Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Reported-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 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@xxxxxxxxxx are

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





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