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

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Date:   Mon, 1 Nov 2021 09:37:52 +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 Fri 29-10-21 09:07:39, Suren Baghdasaryan wrote:
> On Fri, Oct 29, 2021 at 6:03 AM Michal Hocko <mhocko@...e.com> wrote:
[...]
> > Well, I still do not see why that is a problem. This syscall is meant to
> > release the address space not to do it fast.
> 
> It's the same problem for a userspace memory reaper as for the
> oom-reaper. The goal is to release the memory of the victim and to
> quickly move on to the next one if needed.

The purpose of the oom_reaper is to _guarantee_ a forward progress. It
doesn't have to be quick or optimized for speed.
 
[...]

> > Btw. the above code will not really tell you much on a larger machine
> > unless you manage to trigger mmap_sem contection. Otherwise you are
> > measuring the mmap_sem writelock fast path and that should be really
> > within a noise comparing to the whole address space destruction time. If
> > that is not the case then we have a real problem with the locking...
> 
> My understanding of that discussion is that the concern was that even
> taking uncontended mmap_sem writelock would regress the exit path.
> That was what I wanted to confirm. Am I misreading it?

No, your reading match my recollection. I just think that code
robustness in exchange of a rw semaphore write lock fast path is a
reasonable price to pay even if that has some effect on micro
benchmarks.
-- 
Michal Hocko
SUSE Labs