Re: [PATCH v1] mm/vmscan: mitigate spurious kswapd_failures reset from direct reclaim

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 22 Dec 2025 20:20:21 +0800 Jiayuan Chen <jiayuan.chen@linux.dev> wrote:

> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> 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, pods on this machine have memory.high set in their cgroup.

What's a "pod"?

> 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 result is that kswapd runs endlessly, repeatedly evicting the few
> remaining file pages which are actually hot. These pages constantly
> refault, generating sustained heavy IO READ pressure.

Yes, not good.

> Fix this by only resetting kswapd_failures from direct reclaim when the
> node is actually balanced. This prevents direct reclaim from keeping
> kswapd alive when the node cannot be balanced through reclaim alone.
>
> ...
>
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2648,6 +2648,15 @@ static bool can_age_anon_pages(struct lruvec *lruvec,
>  			  lruvec_memcg(lruvec));
>  }
>  
> +static bool pgdat_balanced(pg_data_t *pgdat, int order, int highest_zoneidx);

Forward declaration could be avoided by relocating pgdat_balanced(),
although the patch will get a lot larger.

> +static inline void reset_kswapd_failures(struct pglist_data *pgdat,
> +					 struct scan_control *sc)

It would be nice to have a nice comment explaining why this is here. 
Why are we checking for balanced?

> +{
> +	if (!current_is_kswapd() &&

kswapd can no longer clear ->kswapd_failures.  What's the thinking here?

> +	    pgdat_balanced(pgdat, sc->order, sc->reclaim_idx))
> +		atomic_set(&pgdat->kswapd_failures, 0);
> +}
> +
>  #ifdef CONFIG_LRU_GEN
>  
>  #ifdef CONFIG_LRU_GEN_ENABLED
> @@ -5065,7 +5074,7 @@ 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);
> +		reset_kswapd_failures(pgdat, sc);
>  }
>  
>  /******************************************************************************
> @@ -6139,7 +6148,7 @@ 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);
> +		reset_kswapd_failures(pgdat, sc);
>  	else if (sc->cache_trim_mode)
>  		sc->cache_trim_mode_failed = 1;
>  }