Re: [RFC PATCH 0/1] mm: Reduce direct reclaim stalls with RAM-backed swap

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

Hi Matt,

Thanks for the report and one request I have is to avoid cover letter for a
single patch to avoid partitioning the discussion.

On Tue, Mar 03, 2026 at 11:53:57AM +0000, Matt Fleming wrote:
> From: Matt Fleming <mfleming@cloudflare.com>
> 
> Hi,
> 
> Systems with zram-only swap can spin in direct reclaim for 20-30
> minutes without ever invoking the OOM killer. We've hit this repeatedly
> in production on machines with 377 GiB RAM and a 377 GiB zram device.
> 

Have you tried zswap and if you see similar issues with zswap?

> The problem
> -----------
> 
> should_reclaim_retry() calls zone_reclaimable_pages() to estimate how
> much memory is still reclaimable. That estimate includes anonymous
> pages, on the assumption that swapping them out frees physical pages.
> 
> With disk-backed swap, that's true -- writing a page to disk frees a
> page of RAM, and SwapFree accurately reflects how many more pages can
> be written. With zram, the free slot count is inaccurate. A 377 GiB
> zram device with 10% used reports ~340 GiB of free swap slots, but
> filling those slots requires physical RAM that the system doesn't have
> -- that's why it's in direct reclaim in the first place.
> 
> The reclaimable estimate is off by orders of magnitude.
> 

Over the time we (kernel MM community) have implicitly decided to keep the
kernel oom-killer very conservative as adding more heuristics in the reclaim/oom
path makes the kernel more unreliable and punt the aggressiveness of oom-killing
to the userspace as a policy. All major Linux deployments have started using
userspace oom-killers like systemd-oomd, Android's LMKD, fb-oomd or some
internal alternatives. That provides more flexibility to define the
aggressiveness of oom-killing based on your business needs.

Though userspace oom-killers are prone to reliability issues (oom-killer getting
stuck in reclaim or not getting enough CPU), so we (Roman) are working on adding
support for BPF based oom-killer where wen think we can do oom policies more
reliably.

Anyways, I am wondering if you have tried systemd-oomd or some userspace
alternative. If you are interested in BPF oom-killer, we can help with that as
well.

thanks,
Shakeel