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mm/mglru: fix ineffective protection calculation

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.

Yu Zhao committed 2 years ago

Commit 30d77b71 parent b749cb01 file · +38 −44Browse files

Unified · Split

mm/vmscan.c+38−44
@@ -3915,6 +3915,32 @@ done: · 3914 unmodified lines
39153915 * working set protection
39163916 ******************************************************************************/
39173917
3918+static void set_initial_priority(struct pglist_data *pgdat, struct scan_control *sc)
3919+{
3920+ int priority;
3921+ unsigned long reclaimable;
3922+
3923+ if (sc->priority != DEF_PRIORITY || sc->nr_to_reclaim < MIN_LRU_BATCH)
3924+ return;
3925+ /*
3926+ * Determine the initial priority based on
3927+ * (total >> priority) * reclaimed_to_scanned_ratio = nr_to_reclaim,
3928+ * where reclaimed_to_scanned_ratio = inactive / total.
3929+ */
3930+ reclaimable = node_page_state(pgdat, NR_INACTIVE_FILE);
3931+ if (can_reclaim_anon_pages(NULL, pgdat->node_id, sc))
3932+ reclaimable += node_page_state(pgdat, NR_INACTIVE_ANON);
3933+
3934+ /* round down reclaimable and round up sc->nr_to_reclaim */
3935+ priority = fls_long(reclaimable) - 1 - fls_long(sc->nr_to_reclaim - 1);
3936+
3937+ /*
3938+ * The estimation is based on LRU pages only, so cap it to prevent
3939+ * overshoots of shrinker objects by large margins.
3940+ */
3941+ sc->priority = clamp(priority, DEF_PRIORITY / 2, DEF_PRIORITY);
3942+}
3943+
39183944 static bool lruvec_is_sizable(struct lruvec *lruvec, struct scan_control *sc)
39193945 {
39203946 int gen, type, zone;
@@ -3948,19 +3974,17 @@ static bool lruvec_is_reclaimable(struct lruvec *lruvec, struct scan_control *sc · 27 unmodified lines
39483974 struct mem_cgroup *memcg = lruvec_memcg(lruvec);
39493975 DEFINE_MIN_SEQ(lruvec);
39503976
3951− /* see the comment on lru_gen_folio */
3952− gen = lru_gen_from_seq(min_seq[LRU_GEN_FILE]);
3953− birth = READ_ONCE(lruvec->lrugen.timestamps[gen]);
3954−
3955− if (time_is_after_jiffies(birth + min_ttl))
3977+ if (mem_cgroup_below_min(NULL, memcg))
39563978 return false;
39573979
39583980 if (!lruvec_is_sizable(lruvec, sc))
39593981 return false;
39603982
3961− mem_cgroup_calculate_protection(NULL, memcg);
3983+ /* see the comment on lru_gen_folio */
3984+ gen = lru_gen_from_seq(min_seq[LRU_GEN_FILE]);
3985+ birth = READ_ONCE(lruvec->lrugen.timestamps[gen]);
39623986
3963− return !mem_cgroup_below_min(NULL, memcg);
3987+ return time_is_before_jiffies(birth + min_ttl);
39643988 }
39653989
39663990 /* 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) · 3 unmodified lines
39703994 {
39713995 struct mem_cgroup *memcg;
39723996 unsigned long min_ttl = READ_ONCE(lru_gen_min_ttl);
3997+ bool reclaimable = !min_ttl;
39733998
39743999 VM_WARN_ON_ONCE(!current_is_kswapd());
39754000
3976− /* check the order to exclude compaction-induced reclaim */
3977− if (!min_ttl || sc->order || sc->priority == DEF_PRIORITY)
3978− return;
4001+ set_initial_priority(pgdat, sc);
39794002
39804003 memcg = mem_cgroup_iter(NULL, NULL, NULL);
39814004 do {
39824005 struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
39834006
3984− if (lruvec_is_reclaimable(lruvec, sc, min_ttl)) {
3985− mem_cgroup_iter_break(NULL, memcg);
3986− return;
3987− }
4007+ mem_cgroup_calculate_protection(NULL, memcg);
39884008
3989− cond_resched();
4009+ if (!reclaimable)
4010+ reclaimable = lruvec_is_reclaimable(lruvec, sc, min_ttl);
39904011 } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)));
39914012
39924013 /*
@@ -3994,7 +4015,7 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc) · 1 unmodified line
39944015 * younger than min_ttl. However, another possibility is all memcgs are
39954016 * either too small or below min.
39964017 */
3997− if (mutex_trylock(&oom_lock)) {
4018+ if (!reclaimable && mutex_trylock(&oom_lock)) {
39984019 struct oom_control oc = {
39994020 .gfp_mask = sc->gfp_mask,
40004021 };
@@ -4786,8 +4807,7 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc) · 785 unmodified lines
47864807 struct mem_cgroup *memcg = lruvec_memcg(lruvec);
47874808 struct pglist_data *pgdat = lruvec_pgdat(lruvec);
47884809
4789− mem_cgroup_calculate_protection(NULL, memcg);
4790−
4810+ /* lru_gen_age_node() called mem_cgroup_calculate_protection() */
47914811 if (mem_cgroup_below_min(NULL, memcg))
47924812 return MEMCG_LRU_YOUNG;
47934813
@@ -4911,32 +4931,6 @@ static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc · 117 unmodified lines
49114931 blk_finish_plug(&plug);
49124932 }
49134933
4914−static void set_initial_priority(struct pglist_data *pgdat, struct scan_control *sc)
4915−{
4916− int priority;
4917− unsigned long reclaimable;
4918−
4919− if (sc->priority != DEF_PRIORITY || sc->nr_to_reclaim < MIN_LRU_BATCH)
4920− return;
4921− /*
4922− * Determine the initial priority based on
4923− * (total >> priority) * reclaimed_to_scanned_ratio = nr_to_reclaim,
4924− * where reclaimed_to_scanned_ratio = inactive / total.
4925− */
4926− reclaimable = node_page_state(pgdat, NR_INACTIVE_FILE);
4927− if (can_reclaim_anon_pages(NULL, pgdat->node_id, sc))
4928− reclaimable += node_page_state(pgdat, NR_INACTIVE_ANON);
4929−
4930− /* round down reclaimable and round up sc->nr_to_reclaim */
4931− priority = fls_long(reclaimable) - 1 - fls_long(sc->nr_to_reclaim - 1);
4932−
4933− /*
4934− * The estimation is based on LRU pages only, so cap it to prevent
4935− * overshoots of shrinker objects by large margins.
4936− */
4937− sc->priority = clamp(priority, DEF_PRIORITY / 2, DEF_PRIORITY);
4938−}
4939−
49404934 static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *sc)
49414935 {
49424936 struct blk_plug plug;