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

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Date:   Wed, 27 Oct 2021 11:00:38 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Liam R. Howlett" <liam.howlett@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        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 Wed, Oct 27, 2021 at 10:52 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Wed, Oct 27, 2021 at 10:42:29AM -0700, Suren Baghdasaryan wrote:
> > On Wed, Oct 27, 2021 at 10:35 AM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Wed, Oct 27, 2021 at 09:08:21AM -0700, Suren Baghdasaryan wrote:
> > > > Unconditional mmap_write_lock around free_pgtables in exit_mmap seems
> > > > to me the most semantically correct way forward and the pushback is on
> > > > the basis of regressing performance of the exit path. I would like to
> > > > measure that regression to confirm this. I don't have access to a big
> > > > machine but will ask someone in another Google team to try the test
> > > > Michal wrote here
> > > > https://lore.kernel.org/all/20170725142626.GJ26723@dhcp22.suse.cz/ on
> > > > a server with and without a custom patch.
> > >
> > > Sorry to hijack this, but could you ask that team to also test this
> > > patch?  I think there's probably a good-sized win here, but I have no
> > > profiles to share at this point.  I've only done light testing, and
> > > it may have bugs.
> > >
> > > NB: I only did the exit() path here.  fork() conversion is left as an
> > > exercise for the reader^W^W Liam.
> >
> > To clarify, this patch does not change the mmap_write_lock portion of
> > exit_mmap. Do you want to test it in isolation or with the locking
> > changes in exit_mmap I mentioned?
>
> Correct, it does not.  I think it's interesting to test it in isolation,
> but if you want to test it in in combination, that could also be
> interesting (see if we regain some of the expected performance loss).
> I just don't have a NUMA box of my own to test on, so I'm hoping to
> exploit your test infrastructure ;-)
>
> By the way, my vmavec patch should also be helpful on small systems
> like phones ... ;-)

Sounds good. I'll try to queue up the patches so that it's easy to
test them both in isolation and together.