Re: [PATCH 2/3] mm: drop oom code from exit_mmap

[Suren Baghdasaryan <surenb@google.com>]

On Tue, May 10, 2022 at 1:53 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 10-05-22 09:31:50, Suren Baghdasaryan wrote:
> > On Tue, May 10, 2022 at 6:06 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Mon 09-05-22 20:00:13, Suren Baghdasaryan wrote:
> > > > With the oom-killer being able to operate on locked pages, exit_mmap
> > > > does not need to ensure that oom_reap_task_mm is done before it can
> > > > proceed. Instead it can rely on mmap_lock write lock to prevent
> > > > oom-killer from operating on the vma tree while it's freeing page
> > > > tables. exit_mmap can hold mmap_lock read lock when unmapping vmas
> > > > and then take mmap_lock write lock before freeing page tables.
> > >
> > > The changelog is rather light on nasty details which might be good but
> > > for the sake of our future us let's be more verbose so that we do not
> > > have to reinvent the prior history each time we are looking into this
> > > code. I would go with something like this instead:
> > > "
> > > The primary reason to invoke the oom reaper from the exit_mmap path used
> > > to be a prevention of an excessive oom killing if the oom victim exit
> > > races with the oom reaper (see 212925802454 ("mm: oom: let oom_reap_task
> > > and exit_mmap run concurrently") for more details. The invocation has
> > > moved around since then because of the interaction with the munlock
> > > logic but the underlying reason has remained the same (see 27ae357fa82b
> > > ("mm, oom: fix concurrent munlock and oom reaper unmap, v3").
> > >
> > > Munlock code is no longer a problem since a213e5cf71cb ("mm/munlock:
> > > delete munlock_vma_pages_all(), allow oomreap") and there shouldn't be
> > > any blocking operation before the memory is unmapped by exit_mmap so
> > > the oom reaper invocation can be dropped. The unmapping part can be done
> > > with the non-exclusive mmap_sem and the exclusive one is only required
> > > when page tables are freed.
> > >
> > > Remove the oom_reaper from exit_mmap which will make the code easier to
> > > read. This is really unlikely to make any observable difference although
> > > some microbenchmarks could benefit from one less branch that needs to be
> > > evaluated even though it almost never is true.
> > > "
> >
> > Looks great! Thanks for collecting all the history. Will update the description.
>
> Please make sure you double check the story. This is mostly my
> recollection and brief reading through the said commits. I might
> misremember here and there.

Will do. Thanks!

> --
> Michal Hocko
> SUSE Labs