[PATCH 3/8] mm/mglru: restructure the reclaim loop

Kairui Song via B4 Relay
2026-03-17T19:12:02Z

From: Kairui Song <kasong@tencent.com>

The current loop will calculate the scan number on each iteration. The
number of folios to scan is based on the LRU length, with some unclear
behaviors, eg, it only shifts the scan number by reclaim priority at the
default priority, and it couples the number calculation with aging and
rotation.

Adjust, simplify it, and decouple aging and rotation. Just calculate the
scan number for once at the beginning of the reclaim, always respect the
reclaim priority, and make the aging and rotation more explicit.

This slightly changes how offline memcg aging works: previously, offline
memcg wouldn't be aged unless it didn't have any evictable folios. Now,
we might age it if it has only 3 generations and the reclaim priority is
less than DEF_PRIORITY, which should be fine. On one hand, offline memcg
might still hold long-term folios, and in fact, a long-existing offline
memcg must be pinned by some long-term folios like shmem. These folios
might be used by other memcg, so aging them as ordinary memcg doesn't
seem wrong. And besides, aging enables further reclaim of an offlined
memcg, which will certainly happen if we keep shrinking it. And offline
memcg might soon be no longer an issue once reparenting is all ready.

Overall, the memcg LRU rotation, as described in mmzone.h,
remains the same.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
mm/vmscan.c | 74 ++++++++++++++++++++++++++++++-------------------------------
1 file changed, 36 insertions(+), 38 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d48074f9bd87..ed5b5f8dd3c7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4926,49 +4926,35 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
}

static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
- int swappiness, unsigned long *nr_to_scan)
+ struct scan_control *sc, int swappiness)
{
DEFINE_MIN_SEQ(lruvec);

- *nr_to_scan = 0;
/* have to run aging, since eviction is not possible anymore */
if (evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS > max_seq)
return true;

- *nr_to_scan = lruvec_evictable_size(lruvec, swappiness);
+ /* try to get away with not aging at the default priority */
+ if (sc->priority == DEF_PRIORITY)
+ return false;
+
/* better to run aging even though eviction is still possible */
return evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS == max_seq;
}

-/*
- * For future optimizations:
- * 1. Defer try_to_inc_max_seq() to workqueues to reduce latency for memcg
- * reclaim.
- */
-static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc, int swappiness)
+static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc,
+ struct mem_cgroup *memcg, int swappiness)
{
- bool need_aging;
unsigned long nr_to_scan;
- struct mem_cgroup *memcg = lruvec_memcg(lruvec);
- DEFINE_MAX_SEQ(lruvec);
-
- if (mem_cgroup_below_min(sc->target_mem_cgroup, memcg))
- return -1;
-
- need_aging = should_run_aging(lruvec, max_seq, swappiness, &nr_to_scan);

+ nr_to_scan = lruvec_evictable_size(lruvec, swappiness);
/* try to scrape all its memory if this memcg was deleted */
- if (nr_to_scan && !mem_cgroup_online(memcg))
+ if (!mem_cgroup_online(memcg))
return nr_to_scan;

nr_to_scan = apply_proportional_protection(memcg, sc, nr_to_scan);
-
- /* try to get away with not aging at the default priority */
- if (!need_aging || sc->priority == DEF_PRIORITY)
- return nr_to_scan >> sc->priority;
-
- /* stop scanning this lruvec as it's low on cold folios */
- return try_to_inc_max_seq(lruvec, max_seq, swappiness, false) ? -1 : 0;
+ /* always respect scan priority */
+ return nr_to_scan >> sc->priority;
}

static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
@@ -4998,31 +4984,43 @@ static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
return true;
}

+/*
+ * For future optimizations:
+ * 1. Defer try_to_inc_max_seq() to workqueues to reduce latency for memcg
+ * reclaim.
+ */
static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
{
+ bool need_rotate = false;
long nr_batch, nr_to_scan;
- unsigned long scanned = 0;
int swappiness = get_swappiness(lruvec, sc);
+ struct mem_cgroup *memcg = lruvec_memcg(lruvec);

- while (true) {
+ nr_to_scan = get_nr_to_scan(lruvec, sc, memcg, swappiness);
+ while (nr_to_scan > 0) {
int delta;
+ DEFINE_MAX_SEQ(lruvec);

- nr_to_scan = get_nr_to_scan(lruvec, sc, swappiness);
- if (nr_to_scan <= 0)
+ if (mem_cgroup_below_min(sc->target_mem_cgroup, memcg)) {
+ need_rotate = true;
break;
+ }
+
+ if (should_run_aging(lruvec, max_seq, sc, swappiness)) {
+ if (try_to_inc_max_seq(lruvec, max_seq, swappiness, false))
+ need_rotate = true;
+ break;
+ }

nr_batch = min(nr_to_scan, MAX_LRU_BATCH);
delta = evict_folios(nr_batch, lruvec, sc, swappiness);
if (!delta)
break;

- scanned += delta;
- if (scanned >= nr_to_scan)
- break;
-
if (should_abort_scan(lruvec, sc))
break;

+ nr_to_scan -= delta;
cond_resched();
}

@@ -5034,12 +5032,12 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
wakeup_flusher_threads(WB_REASON_VMSCAN);

/* whether this lruvec should be rotated */
- return nr_to_scan < 0;
+ return need_rotate;
}

static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
{
- bool success;
+ bool need_rotate;
unsigned long scanned = sc->nr_scanned;
unsigned long reclaimed = sc->nr_reclaimed;
struct mem_cgroup *memcg = lruvec_memcg(lruvec);
@@ -5057,7 +5055,7 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
memcg_memory_event(memcg, MEMCG_LOW);
}

- success = try_to_shrink_lruvec(lruvec, sc);
+ need_rotate = try_to_shrink_lruvec(lruvec, sc);

shrink_slab(sc->gfp_mask, pgdat->node_id, memcg, sc->priority);

@@ -5067,10 +5065,10 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)

flush_reclaim_state(sc);

- if (success && mem_cgroup_online(memcg))
+ if (need_rotate && mem_cgroup_online(memcg))
return MEMCG_LRU_YOUNG;

- if (!success && lruvec_is_sizable(lruvec, sc))
+ if (!need_rotate && lruvec_is_sizable(lruvec, sc))
return 0;

/* one retry if offlined or too small */

--
2.53.0