Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

From: Suren Baghdasaryan <surenb@google.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
	Minchan Kim <minchan@kernel.org>,
	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>,
	Linux API <linux-api@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap
Date: Tue, 9 Nov 2021 11:37:06 -0800	[thread overview]
Message-ID: <CAJuCfpG0d34yRhuvOj9NX9zMp=6jWLqFPfUGV0sOO6OrwNC89A@mail.gmail.com> (raw)
In-Reply-To: <YYrLe2u2zbmu4LfL@dhcp22.suse.cz>

On Tue, Nov 9, 2021 at 11:26 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 09-11-21 11:01:02, Suren Baghdasaryan wrote:
> [...]
> > Discussing how the patch I want to post works for maple trees that
> > Matthew is working on, I've got a question:
> >
> > IIUC, according to Michal's post here:
> > https://lore.kernel.org/all/20170725154514.GN26723@dhcp22.suse.cz,
> > unmap_vmas() can race with other mmap_lock read holders (including
> > oom_reap_task_mm()) with no issues.
> > Maple tree patchset requires rcu read lock or the mmap semaphore be
> > held (read or write side) when walking the tree, including inside
> > unmap_vmas(). When asked, he told me that he is not sure why it's
> > currently "safe" to walk the vma->vm_next list in unmap_vmas() while
> > another thread is reaping the mm.
> > Michal (or maybe someone else), could you please clarify why
> > unmap_vmas() can safely race with oom_reap_task_mm()? Or maybe my
> > understanding was wrong?
>
> I cannot really comment on the mapple tree part. But the existing
> synchronization between oom reaper and exit_mmap is based on
> - oom_reaper takes mmap_sem for reading
> - exit_mmap sets MMF_OOM_SKIP and takes the exclusive mmap_sem before
>   unmap_vmas.
>
> The oom_reaper therefore can either unmap the address space if the lock
> is taken before exit_mmap or it would it would bale out on MMF_OOM_SKIP
> if it takes the lock afterwards. So the reaper cannot race with
> unmap_vmas.

I see. So, it's the combination of MMF_OOM_SKIP and mmap_lock working
as a barrier which prevent them from racing with each other...
I wasn't sure how
https://lore.kernel.org/all/20170724072332.31903-1-mhocko@kernel.org/
was implementing this synchronization because it would take mmap_sem
write side after unmap_vmas() and IIUC there was no
"mmap_lock_write(); mmap_unlock_write();" sequence in exit_mmap at
that time. I'll need to checkout the old sources to figure this out.
Thanks,
Suren.

>
> --
> Michal Hocko
> SUSE Labs