Re: [PATCH] mm: Mark the OOM reaper thread as freezable

From: Sultan Alsawaf <sultan@kerneltoast.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Mel Gorman <mgorman@suse.de>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: Mark the OOM reaper thread as freezable
Date: Mon, 20 Sep 2021 15:29:46 -0700	[thread overview]
Message-ID: <YUkLWuJYrvNp+tMh@sultan-box.localdomain> (raw)
In-Reply-To: <YUjvVP8pFL3Be9Jv@dhcp22.suse.cz>

On Mon, Sep 20, 2021 at 10:30:12PM +0200, Michal Hocko wrote:
> We usually tend to provide Fixes where there has been something fixed.
> It seems just confusing if it is used for non functional changes,
> cleanups etc. Thera are gray zones of course.

Got it, thanks. So no tag would be used in such a case?

> I am not sure I follow. My understanding is that we need to make sure
> oom_reaper is not running after the quiescent state as it is changing
> user space address space. For that I believe we need to freeze the
> kthread at a proper moment. That is currently the entry point and maybe
> we can extend that even to the reaping loop but I haven't really
> explored that. PF_FREEZER_SKIP would skip over the reaper and that could
> result in it racing with the snapshotting no?

Kthreads cannot be implicitly frozen; it's not like PREEMPT. From what I've read
in the freezer code, two things must occur for a kthread to freeze: the kthread
must have PF_NOFREEZE unset (via set_freezable(), as is done in the patch I've
submitted here), and the kthread must have an explicit call into the freezer,
such as via wait_event_freezable().

Right now, oom_reaper is totally ignored by the freezer because PF_NOFREEZE is
set by default in all kthreads. As such, oom_reaper can keep running while
system-wide freezing occurs. If you think this can mangle snapshots, then
perhaps there is a real bug here after all.

It sounds like you don't want oom_reaper to slow down system-wide freezing, but
at the same time, you want oom_reaper to participate in system-wide freezing?
I'm not sure how you could achieve that, aside from maybe inserting a call into
the freezer while iterating through each vma, akin to adding a cond_resched().

My PF_FREEZER_SKIP suggestion was just to emphasize that oom_reaper is currently
skipping the freezer anyway due to PF_NOFREEZE, and that you could set
PF_FREEZER_SKIP to make it skip the freezer a little faster if you wanted.

> Is this something to really worry about?

I'm trying to emphasize that the current usage of wait_event_freezable() in
oom_repear behaves *exactly* like wait_event_interruptible() but with some extra
overhead.

Sultan