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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 4 Jan 2022 14:24:50 -0800
Message-ID: <CAJuCfpFm-4GSu4D_tWMV0a08LNVLjksEnRgRfaYvbciK=wCScA@mail.gmail.com>
Subject: Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap
To:     Hugh Dickins <hughd@google.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
        Minchan Kim <minchan@kernel.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Christian Brauner <christian@brauner.io>,
        Christoph Hellwig <hch@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Jann Horn <jannh@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Florian Weimer <fweimer@redhat.com>,
        Jan Engelhardt <jengelh@inai.de>,
        Tim Murray <timmurray@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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On Mon, Jan 3, 2022 at 1:16 PM Hugh Dickins <hughd@google.com> wrote:
>
> On Mon, 3 Jan 2022, Michal Hocko wrote:
> > On Thu 30-12-21 09:29:40, Suren Baghdasaryan wrote:
> > > On Thu, Dec 30, 2021 at 12:24 AM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > It would be really great to make unlock_range oom reaper aware IMHO.
> > >
> > > What exactly do you envision? Say unlock_range() knows that it's
> > > racing with __oom_reap_task_mm() and that calling follow_page() is
> > > unsafe without locking, what should it do?
> >
> > My original plan was to make the page lock conditional and use
> > trylocking from the oom reaper (aka lockless context). It is OK to
> > simply bail out and leave some mlocked memory behind if there is a
> > contention on a specific page. The overall objective is to free as much
> > memory as possible, not all of it.
> >
> > IIRC Hugh was not a fan of this approach and he has mentioned that the
> > lock might not be even really needed and that the area would benefit
> > from a clean up rather than oom reaper specific hacks. I do tend to
> > agree with that. I just never managed to find any spare time for that
> > though and heavily mlocked oom victims tend to be really rare.
>
> I forget when that was, and what I had in mind at that time.
> But yes, by now I am very sure that munlocking needs a cleanup.
>
> And I do have that cleanup (against a much older tree), but never
> the time to rebase or post or shepherd it through N revisions.

How old was that tree?

>
> It was 22 files changed, 464 insertions, 706 deletions:
> which is too much to help with this immediate oom reaper question.
>
> I'd better not drive this discussion further off-course; but it pains
> me to see munlock_vma_pages obstructing, knowing there's a better way.
>
> I wonder: what if I were to steal time and promise to post a
> rebased series against 5.17-rc1 or rc2: not support it thereafter,
> but might there be someone to pick it up and shepherd it through?
> But there's no answer to that, without you seeing what it's like.

I would be interested in taking a look and see if it can be upstreamed
and supported without bugging you too much.
Thanks,
Suren.

>
> Hugh