linux-kernel - Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit

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Message-ID: <YbIzOR13uPOI5EMB@dhcp22.suse.cz>
Date:   Thu, 9 Dec 2021 17:47:53 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, rientjes@...gle.com,
        willy@...radead.org, hannes@...xchg.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, kirill@...temov.name,
        aarcange@...hat.com, christian@...uner.io, hch@...radead.org,
        oleg@...hat.com, david@...hat.com, jannh@...gle.com,
        shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
        fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap

On Thu 09-12-21 08:24:04, Suren Baghdasaryan wrote:
> On Thu, Dec 9, 2021 at 1:12 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > Do we want this on top?
> 
> As we discussed in this thread
> https://lore.kernel.org/all/YY4snVzZZZYhbigV@dhcp22.suse.cz,
> __oom_reap_task_mm in exit_mmap allows oom-reaper/process_mrelease to
> unmap pages in parallel with exit_mmap without blocking each other.
> Removal of __oom_reap_task_mm from exit_mmap prevents this parallelism
> and has a negative impact on performance. So the conclusion of that
> thread I thought was to keep that part. My understanding is that we
> also wanted to remove MMF_OOM_SKIP as a follow-up patch but
> __oom_reap_task_mm would stay.

OK, then we were talking past each other, I am afraid. I really wanted
to get rid of this oom specific stuff from exit_mmap. It was there out
of necessity. With a proper locking we can finally get rid of the crud.
As I've said previously oom reaping has never been a hot path.

If we really want to optimize this path then I would much rather see a
generic solution which would allow to move the write lock down after
unmap_vmas. That would require oom reaper to be able to handle mlocked
memory.
-- 
Michal Hocko
SUSE Labs