Re: [PATCH v2] mm/vmscan: mitigate spurious kswapd_failures reset from direct reclaim

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Dec 26, 2025 at 04:00:42PM +0800, Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> This is v2 of this patch series. For v1, see [1].
> 
> 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).
> 
> [1] https://lore.kernel.org/all/20251222122022.254268-1-jiayuan.chen@linux.dev/
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>

Hi Jiayuan, can you please send v3 of this patch with the following
additional information:

1. Impact of the patch on your production jobs i.e. does it really
solves the issue?

2. Memory reclaim stats or cpu usage of kswapd with and without patch.

thanks,
Shakeel