From: Jiayuan Chen When kswapd fails to reclaim memory, kswapd_failures is incremented. Once it reaches MAX_RECLAIM_RETRIES, kswapd stops running to avoid futile reclaim attempts. However, any successful direct reclaim unconditionally resets kswapd_failures to 0, which can cause problems. We observed an issue in production on a multi-NUMA system where a process allocated large amounts of anonymous pages on a single NUMA node, causing its watermark to drop below high and evicting most file pages: $ numastat -m Per-node system memory usage (in MBs): Node 0 Node 1 Total --------------- --------------- --------------- MemTotal 128222.19 127983.91 256206.11 MemFree 1414.48 1432.80 2847.29 MemUsed 126807.71 126551.11 252358.82 SwapCached 0.00 0.00 0.00 Active 29017.91 25554.57 54572.48 Inactive 92749.06 95377.00 188126.06 Active(anon) 28998.96 23356.47 52355.43 Inactive(anon) 92685.27 87466.11 180151.39 Active(file) 18.95 2198.10 2217.05 Inactive(file) 63.79 7910.89 7974.68 With swap disabled, only file pages can be reclaimed. When kswapd is woken (e.g., via wake_all_kswapds()), it runs continuously but cannot raise free memory above the high watermark since reclaimable file pages are insufficient. Normally, kswapd would eventually stop after kswapd_failures reaches MAX_RECLAIM_RETRIES. However, containers on this machine have memory.high set in their cgroup. Business processes continuously trigger the high limit, causing frequent direct reclaim that keeps resetting kswapd_failures to 0. This prevents kswapd from ever stopping. The key insight is that direct reclaim triggered by cgroup memory.high performs aggressive scanning to throttle the allocating process. With sufficiently aggressive scanning, even hot pages will eventually be reclaimed, making direct reclaim "successful" at freeing some memory. However, this success does not mean the node has reached a balanced state - the freed memory may still be insufficient to bring free pages above the high watermark. Unconditionally resetting kswapd_failures in this case keeps kswapd alive indefinitely. The result is that kswapd runs endlessly. Unlike direct reclaim which only reclaims from the allocating cgroup, kswapd scans the entire node's memory. This causes hot file pages from all workloads on the node to be evicted, not just those from the cgroup triggering memory.high. These pages constantly refault, generating sustained heavy IO READ pressure across the entire system. Fix this by only resetting kswapd_failures when the node is actually balanced. This allows both kswapd and direct reclaim to clear kswapd_failures upon successful reclaim, but only when the reclaim actually resolves the memory pressure (i.e., the node becomes balanced). Signed-off-by: Jiayuan Chen Signed-off-by: Jiayuan Chen --- mm/vmscan.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 670fe9fae5ba..6fd100130987 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2650,6 +2650,25 @@ static bool can_age_anon_pages(struct lruvec *lruvec, lruvec_memcg(lruvec)); } +static void pgdat_reset_kswapd_failures(pg_data_t *pgdat) +{ + atomic_set(&pgdat->kswapd_failures, 0); +} + +/* + * Reset kswapd_failures only when the node is balanced. Without this + * check, successful direct reclaim (e.g., from cgroup memory.high + * throttling) can keep resetting kswapd_failures even when the node + * cannot be balanced, causing kswapd to run endlessly. + */ +static bool pgdat_balanced(pg_data_t *pgdat, int order, int highest_zoneidx); +static inline void pgdat_try_reset_kswapd_failures(struct pglist_data *pgdat, + struct scan_control *sc) +{ + if (pgdat_balanced(pgdat, sc->order, sc->reclaim_idx)) + pgdat_reset_kswapd_failures(pgdat); +} + #ifdef CONFIG_LRU_GEN #ifdef CONFIG_LRU_GEN_ENABLED @@ -5067,7 +5086,7 @@ static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control * blk_finish_plug(&plug); done: if (sc->nr_reclaimed > reclaimed) - atomic_set(&pgdat->kswapd_failures, 0); + pgdat_try_reset_kswapd_failures(pgdat, sc); } /****************************************************************************** @@ -6141,7 +6160,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc) * successful direct reclaim run will revive a dormant kswapd. */ if (reclaimable) - atomic_set(&pgdat->kswapd_failures, 0); + pgdat_try_reset_kswapd_failures(pgdat, sc); else if (sc->cache_trim_mode) sc->cache_trim_mode_failed = 1; } -- 2.43.0