linux-kernel - Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit

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Message-ID: <Ybsn2hJZXRofwuv+@cmpxchg.org>
Date:   Thu, 16 Dec 2021 12:49:46 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>, akpm@...ux-foundation.org,
        rientjes@...gle.com, willy@...radead.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, kirill@...temov.name,
        aarcange@...hat.com, christian@...uner.io, hch@...radead.org,
        oleg@...hat.com, david@...hat.com, jannh@...gle.com,
        shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
        fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap

On Wed, Dec 15, 2021 at 06:26:11PM -0800, Suren Baghdasaryan wrote:
> On Thu, Dec 9, 2021 at 9:06 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> >
> > On Thu, Dec 9, 2021 at 8:47 AM Michal Hocko <mhocko@...e.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@...e.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?

A page's mlock state is determined by whether any of the vmas that map
it are mlocked. The munlock code does:

vma->vm_flags &= VM_LOCKED_CLEAR_MASK
TestClearPageMlocked()
isolate_lru_page()
__munlock_isolated_page()
  page_mlock()
    rmap_walk() # for_each_vma()
      page_mlock_one()
        (vma->vm_flags & VM_LOCKED) && TestSetPageMlocked()

If we didn't clear the VM_LOCKED flag first, racing threads could
re-lock pages under us because they see that flag and think our vma
wants those pages mlocked when we're in the process of munlocking.