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

Axel Rasmussen
2026-03-20T20:10:15Z

This looks like a reasonable refactor to me. To me the new code is
more straightforward to reason about, and I don't see anything this
breaks (either by inspection of with basic functional testing).

Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>

On Tue, Mar 17, 2026 at 12:11 PM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@kernel.org> wrote:
>
> 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 */

It's a nitpick, but with the variable rename, this comment isn't doing
is much good now. :)

> - 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
>
>