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

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Date:   Tue, 21 Sep 2021 09:50:18 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Sultan Alsawaf <sultan@...neltoast.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Mark the OOM reaper thread as freezable

On Mon 20-09-21 15:29:46, Sultan Alsawaf wrote:
> 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.

OK, now I am getting your point. Sorry for being dense here. Process
freezing has always been kinda muddy to me as I've said earlier. I have
completely misunderstood your earlier PF_NOFREEZE remark.
-- 
Michal Hocko
SUSE Labs