Re: [PATCH v4] mm: vmscan: abort proactive reclaim early when freezing for suspend

[Andrew Morton]

On Mon, 20 Jul 2026 04:41:03 +0000 Richard Chang <richardycc@xxxxxxxxxx> wrote:

> Proactive reclaim (triggered via memory.reclaim or node sysfs) checks
> for pending signals in its outer loop in user_proactive_reclaim().
> However, the inner reclaim loops—specifically scanning cgroups in
> shrink_many() and evicting/aging folios in try_to_shrink_lruvec()—can
> run for a long time before returning to the outer loop, especially on
> systems with many cgroups or large memory sizes.
> 
> During system suspend, the PM freezer attempts to freeze all tasks by
> sending fake signals (setting TIF_SIGPENDING). Because the inner loops
> do not check for pending signals, the proactive reclaim task can remain
> stuck in kernel space for seconds, failing to enter the refrigerator in
> a timely manner. This leads to suspend failures due to freeze timeouts,
> a behavior observed on Android devices.
> 
> This latency issue is specific to proactive reclaim because of its
> large, user-defined reclaim targets (could be gigabytes). Since commit
> 287d5fedb377 ("mm: memcg: use larger batches for proactive reclaim"),
> proactive reclaim uses larger decaying batch sizes (starting at 1/4 of
> the remaining target) to maintain throughput. This keeps the task in
> the inner reclaim loop for extended periods. In contrast, reactive
> reclaim (global/memcg) uses small targets (SWAP_CLUSTER_MAX, typically
> 32 pages), allowing it to return to the outer loop and check signals
> frequently.

So 287d5fedb377 led to suspend failures on MGLRU-using kernels.

That's a regression which justifies a Fixes: and a cc:stable, don't
people agree?

AI review asked a couple of serious-sounding questions:
	https://sashiko.dev/#/patchset/20260720044103.905191-1-richardycc@xxxxxxxxxx

> To fix this, add a signal_pending() check to should_abort_scan() for
> proactive reclaim paths. Since should_abort_scan() is called within
> the inner scanning and eviction loops, this allows proactive reclaim to
> abort early and return to the outer loop in user_proactive_reclaim().
> 
> Additionally, return -ERESTARTSYS instead of -EINTR in
> user_proactive_reclaim(). When interrupted by system suspend, returning
> -ERESTARTSYS allows the task to enter the refrigerator and automatically
> restart the syscall upon resume, making the freezer transparent to
> userspace. For real signals, the signal layer will either restart the
> syscall (if SA_RESTART is set) or return -EINTR to userspace.
> 
> This fix specifically targets Multi-Gen LRU (MGLRU). Classic LRU's scan
> targets per iteration are strictly bounded by get_scan_count(), which
> ensures it returns to the outer loop more frequently.
> 
> The check in should_abort_scan() is limited to proactive reclaim
> (sc->proactive) to avoid inadvertently affecting reactive reclaim paths,
> and is wrapped in unlikely() as it is a slow path.
>