Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap

From: Suren Baghdasaryan <surenb@google.com>
To: Michal Hocko <mhocko@suse.com>
Cc: akpm@linux-foundation.org, rientjes@google.com,
	willy@infradead.org, hannes@cmpxchg.org, guro@fb.com,
	riel@surriel.com, minchan@kernel.org, kirill@shutemov.name,
	aarcange@redhat.com, christian@brauner.io, hch@infradead.org,
	oleg@redhat.com, david@redhat.com, jannh@google.com,
	shakeelb@google.com, luto@kernel.org,
	christian.brauner@ubuntu.com, fweimer@redhat.com,
	jengelh@inai.de, timmurray@google.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap
Date: Wed, 15 Dec 2021 18:26:11 -0800	[thread overview]
Message-ID: <CAJuCfpHZOojVXCDXPOA7kgrZqNWw_BzRmbKQXz3FOCHwa-LopQ@mail.gmail.com> (raw)
In-Reply-To: <CAJuCfpHs_25TQ7pHDhPXghA46jmXa5N2OOeBByE-sn+dQ-aM2w@mail.gmail.com>

On Thu, Dec 9, 2021 at 9:06 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Dec 9, 2021 at 8:47 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 09-12-21 08:24:04, Suren Baghdasaryan wrote:
> > > On Thu, Dec 9, 2021 at 1:12 AM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > Do we want this on top?
> > >
> > > As we discussed in this thread
> > > https://lore.kernel.org/all/YY4snVzZZZYhbigV@dhcp22.suse.cz,
> > > __oom_reap_task_mm in exit_mmap allows oom-reaper/process_mrelease to
> > > unmap pages in parallel with exit_mmap without blocking each other.
> > > Removal of __oom_reap_task_mm from exit_mmap prevents this parallelism
> > > and has a negative impact on performance. So the conclusion of that
> > > thread I thought was to keep that part. My understanding is that we
> > > also wanted to remove MMF_OOM_SKIP as a follow-up patch but
> > > __oom_reap_task_mm would stay.
> >
> > OK, then we were talking past each other, I am afraid. I really wanted
> > to get rid of this oom specific stuff from exit_mmap. It was there out
> > of necessity. With a proper locking we can finally get rid of the crud.
> > As I've said previously oom reaping has never been a hot path.
> >
> > If we really want to optimize this path then I would much rather see a
> > generic solution which would allow to move the write lock down after
> > unmap_vmas. That would require oom reaper to be able to handle mlocked
> > memory.
>
> Ok, let's work on that and when that's done we can get rid of the oom
> stuff in exit_mmap. I'll look into this over the weekend and will
> likely be back with questions.

As promised, I have a question:
Any particular reason why munlock_vma_pages_range clears VM_LOCKED
before unlocking pages and not after (see:
https://elixir.bootlin.com/linux/latest/source/mm/mlock.c#L424)? Seems
to me if VM_LOCKED was reset at the end (with proper ordering) then
__oom_reap_task_mm would correctly skip VM_LOCKED vmas.
https://lore.kernel.org/lkml/20180514064824.534798031@linuxfoundation.org/
has this explanation:

"Since munlock_vma_pages_range() depends on clearing VM_LOCKED from
vm_flags before actually doing the munlock to determine if any other
vmas are locking the same memory, the check for VM_LOCKED in the oom
reaper is racy."

but "to determine if any other vmas are locking the same memory"
explanation eludes me... Any insights?
Thanks,
Suren.

> Thanks!
>
> > --
> > Michal Hocko
> > SUSE Labs