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.

Reported-by: T.J. Mercier tjmercier@google.com Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists") Cc: stable@vger.kernel.org Signed-off-by: Yu Zhao yuzhao@google.com --- mm/vmscan.c | 86 +++++++++++++++++++++++++---------------------------- 1 file changed, 40 insertions(+), 46 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c index 6216d79edb7f..525d3ffa8451 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3915,6 +3915,32 @@ static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long seq, * working set protection ******************************************************************************/

+static void set_initial_priority(struct pglist_data *pgdat, struct scan_control *sc) +{ + int priority; + unsigned long reclaimable; + + if (sc->priority != DEF_PRIORITY || sc->nr_to_reclaim < MIN_LRU_BATCH) + return; + /* + * Determine the initial priority based on + * (total >> priority) * reclaimed_to_scanned_ratio = nr_to_reclaim, + * where reclaimed_to_scanned_ratio = inactive / total. + */ + reclaimable = node_page_state(pgdat, NR_INACTIVE_FILE); + if (can_reclaim_anon_pages(NULL, pgdat->node_id, sc)) + reclaimable += node_page_state(pgdat, NR_INACTIVE_ANON); + + /* round down reclaimable and round up sc->nr_to_reclaim */ + priority = fls_long(reclaimable) - 1 - fls_long(sc->nr_to_reclaim - 1); + + /* + * The estimation is based on LRU pages only, so cap it to prevent + * overshoots of shrinker objects by large margins. + */ + sc->priority = clamp(priority, DEF_PRIORITY / 2, DEF_PRIORITY); +} + static bool lruvec_is_sizable(struct lruvec *lruvec, struct scan_control *sc) { int gen, type, zone; @@ -3948,19 +3974,17 @@ static bool lruvec_is_reclaimable(struct lruvec *lruvec, struct scan_control *sc struct mem_cgroup *memcg = lruvec_memcg(lruvec); DEFINE_MIN_SEQ(lruvec);

+ if (mem_cgroup_below_min(NULL, memcg)) + return false; + + if (!lruvec_is_sizable(lruvec, sc)) + return false; + /* 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)) - return false; - - if (!lruvec_is_sizable(lruvec, sc)) - return false; - - mem_cgroup_calculate_protection(NULL, memcg); - - 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 */ @@ -3970,23 +3994,20 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc) { 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)));

/* @@ -3994,7 +4015,7 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc) * 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, }; @@ -4786,8 +4807,7 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc) 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;

@@ -4911,32 +4931,6 @@ static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc blk_finish_plug(&plug); }

-static void set_initial_priority(struct pglist_data *pgdat, struct scan_control *sc) -{ - int priority; - unsigned long reclaimable; - - if (sc->priority != DEF_PRIORITY || sc->nr_to_reclaim < MIN_LRU_BATCH) - return; - /* - * Determine the initial priority based on - * (total >> priority) * reclaimed_to_scanned_ratio = nr_to_reclaim, - * where reclaimed_to_scanned_ratio = inactive / total. - */ - reclaimable = node_page_state(pgdat, NR_INACTIVE_FILE); - if (can_reclaim_anon_pages(NULL, pgdat->node_id, sc)) - reclaimable += node_page_state(pgdat, NR_INACTIVE_ANON); - - /* round down reclaimable and round up sc->nr_to_reclaim */ - priority = fls_long(reclaimable) - 1 - fls_long(sc->nr_to_reclaim - 1); - - /* - * The estimation is based on LRU pages only, so cap it to prevent - * overshoots of shrinker objects by large margins. - */ - sc->priority = clamp(priority, DEF_PRIORITY / 2, DEF_PRIORITY); -} - static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *sc) { struct blk_plug plug;