+ mm-mglru-fix-overshooting-shrinker-memory.patch added to mm-unstable branch - Linux-stable-mirror

11 Jul 2024

The patch titled
     Subject: mm/mglru: fix overshooting shrinker memory
has been added to the -mm mm-unstable branch.  Its filename is
     mm-mglru-fix-overshooting-shrinker-memory.patch
This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Yu Zhao yuzhao@google.com
Subject: mm/mglru: fix overshooting shrinker memory
Date: Thu, 11 Jul 2024 13:19:57 -0600
set_initial_priority() tries to jump-start global reclaim by estimating
the priority based on cold/hot LRU pages.  The estimation does not account
for shrinker objects, and it cannot do so because their sizes can be in
different units other than page.
If shrinker objects are the majority, e.g., on TrueNAS SCALE 24.04.0 where
ZFS ARC can use almost all system memory, set_initial_priority() can
vastly underestimate how much memory ARC shrinker can evict and assign
extreme low values to scan_control->priority, resulting in overshoots of
shrinker objects.
To reproduce the problem, using TrueNAS SCALE 24.04.0 with 32GB DRAM, a
test ZFS pool and the following commands:
fio --name=mglru.file --numjobs=36 --ioengine=io_uring \
      --directory=/root/test-zfs-pool/ --size=1024m --buffered=1 \
      --rw=randread --random_distribution=random \
      --time_based --runtime=1h &
for ((i = 0; i < 20; i++))
  do
    sleep 120
    fio --name=mglru.anon --numjobs=16 --ioengine=mmap \
      --filename=/dev/zero --size=1024m --fadvise_hint=0 \
      --rw=randrw --random_distribution=random \
      --time_based --runtime=1m
  done
To fix the problem:
1. Cap scan_control->priority at or above DEF_PRIORITY/2, to prevent
   the jump-start from being overly aggressive.
2. Account for the progress from mm_account_reclaimed_pages(), to
   prevent kswapd_shrink_node() from raising the priority
   unnecessarily.
Link: https://lkml.kernel.org/r/20240711191957.939105-2-yuzhao@google.com
Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
Signed-off-by: Yu Zhao yuzhao@google.com
Reported-by: Alexander Motin mav@ixsystems.com
Cc: Wei Xu weixugc@google.com
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Morton akpm@linux-foundation.org
---
mm/vmscan.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/mm/vmscan.c~mm-mglru-fix-overshooting-shrinker-memory
+++ a/mm/vmscan.c
@@ -4930,7 +4930,11 @@ static void set_initial_priority(struct
    /* round down reclaimable and round up sc->nr_to_reclaim */
    priority = fls_long(reclaimable) - 1 - fls_long(sc->nr_to_reclaim - 1);
-	sc->priority = clamp(priority, 0, DEF_PRIORITY);
+	/*
+	 * 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)
@@ -6754,6 +6758,7 @@ static bool kswapd_shrink_node(pg_data_t
 {
    struct zone *zone;
    int z;
+	unsigned long nr_reclaimed = sc->nr_reclaimed;
/* Reclaim a number of pages proportional to the number of zones */
    sc->nr_to_reclaim = 0;
@@ -6781,7 +6786,8 @@ static bool kswapd_shrink_node(pg_data_t
    if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order))
    	sc->order = 0;
-	return sc->nr_scanned >= sc->nr_to_reclaim;
+	/* account for progress from mm_account_reclaimed_pages() */
+	return max(sc->nr_scanned, sc->nr_reclaimed - nr_reclaimed) >= sc->nr_to_reclaim;
 }
/* Page allocator PCP high watermark is lowered if reclaim is active. */
_
Patches currently in -mm which might be from yuzhao@google.com are
mm-truncate-batch-clear-shadow-entries.patch
mm-truncate-batch-clear-shadow-entries-v2.patch
mm-mglru-fix-div-by-zero-in-vmpressure_calc_level.patch
mm-mglru-fix-overshooting-shrinker-memory.patch

    