MGLRU premature memcg OOM on slow writes

From: Axel Rasmussen <axelrasmussen@google.com>
To: chris@chrisdown.name
Cc: cgroups@vger.kernel.org, hannes@cmpxchg.org, kernel-team@fb.com,
	 linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	yuzhao@google.com
Subject: MGLRU premature memcg OOM on slow writes
Date: Thu, 29 Feb 2024 15:51:33 -0800	[thread overview]
Message-ID: <20240229235134.2447718-1-axelrasmussen@google.com> (raw)
In-Reply-To: <ZcWOh9u3uqZjNFMa@chrisdown.name>

Hi Chris,

A couple of dumb questions. In your test, do you have any of the following
configured / enabled?

/proc/sys/vm/laptop_mode
memory.low
memory.min

Besides that, it looks like the place non-MGLRU reclaim wakes up the
flushers is in shrink_inactive_list() (which calls wakeup_flusher_threads()).
Since MGLRU calls shrink_folio_list() directly (from evict_folios()), I agree it
looks like it simply will not do this.

Yosry pointed out [1], where MGLRU used to call this but stopped doing that. It
makes sense to me at least that doing writeback every time we age is too
aggressive, but doing it in evict_folios() makes some sense to me, basically to
copy the behavior the non-MGLRU path (shrink_inactive_list()) has.

I can send a patch which tries to implement this next week. In the meantime, Yu,
please let me know if what I've said here makes no sense for some reason. :)

[1]: https://lore.kernel.org/lkml/YzSiWq9UEER5LKup@google.com/