linux-kernel - Re: [PATCH] mm: allow exiting processes to exceed the memory.max limit

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Message-ID: <0623a91b45b8ac96c9b7b910ef3a916f2f924050.camel@surriel.com>
Date: Mon, 09 Dec 2024 15:00:08 -0500
From: Rik van Riel <riel@...riel.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Johannes Weiner <hannes@...xchg.org>, kernel-team@...a.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, Roman Gushchin	
 <roman.gushchin@...ux.dev>, Shakeel Butt <shakeel.butt@...ux.dev>, Muchun
 Song	 <muchun.song@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
	cgroups@...r.kernel.org
Subject: Re: [PATCH] mm: allow exiting processes to exceed the memory.max
 limit

On Mon, 2024-12-09 at 19:08 +0100, Michal Hocko wrote:
> On Mon 09-12-24 12:42:33, Rik van Riel wrote:
> > It is possible for programs to get stuck in exit, when their
> > memcg is at or above the memory.max limit, and things like
> > the do_futex() call from mm_release() need to page memory in.
> > 
> > This can hang forever, but it really doesn't have to.
> 
> Are you sure this is really happening?

It turns out it wasn't really forever.

After about a day the zombie task I was bpftracing,
to figure out exactly what was going wrong, finally
succeeded in exiting.

I got as far as seeing try_to_free_mem_cgroup_pages return 0
many, times in a row, looping in try_charge_memcg, which
occasionally returned -ENOMEM to the caller, who then retried
several times.

Each invocation of try_to_free_mem_cgroup_pages also saw
a large number of unsuccessful calls to shrink_folio_list.

It looks like what might be happening instead is that
faultin_page() returns 0 after getting back VM_FAULT_OOM
from handle_mm_fault, causing __get_user_pages() to loop.

Let me send a patch to fix that, instead!

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