linux-kernel - Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit

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Date:   Fri, 12 Nov 2021 09:58:05 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Christoph Hellwig <hch@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Florian Weimer <fweimer@...hat.com>,
        Jan Engelhardt <jengelh@...i.de>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and
 exit_mmap

On Thu 11-11-21 07:02:42, Suren Baghdasaryan wrote:
> On Thu, Nov 11, 2021 at 1:20 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Wed 10-11-21 17:49:37, Suren Baghdasaryan wrote:
> > > On Tue, Nov 9, 2021 at 1:10 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > >
> > > > On Tue, Nov 9, 2021 at 12:10 PM Michal Hocko <mhocko@...e.com> wrote:
> > [...]
> > > > > Yes, those can run concurrently. One thing I completely forgot about is
> > > > > 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap, v3")
> > > > > which is about interaction with the munlock.
> > >
> > > Agrh! This interaction with the munlock you mentioned requires us to
> > > take mmap_write_lock before munlock_vma_pages_all and that prevents
> > > __oom_reap_task_mm from running concurrently with unmap_vmas. The
> > > reapers would not be as effective as they are now after such a change
> > > :(
> >
> > __oom_reap_task_mm will not run concurrently with unmap_vmas even
> > with the current code. The mmap_sem barrier right before munlock code
> > prevents that.
> 
> You are right, it will run concurrently with another
> __oom_reap_task_mm in the exit_mmap. But I thought we wanted to get
> rid of that call to __oom_reap_task_mm in exit_mmap or did I
> misunderstand?

I do not remember this to be objective or the motivation. IIRC we wanted
to make the locking more robust which would help your process_mrelease
use case. This one currently suffers from a much heavier cost if it
turns out to be the last holder of the reference count on the address
space.
-- 
Michal Hocko
SUSE Labs