linux-kernel - Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

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Message-ID: <CAJuCfpFSD98fw=844AJPy+LT5y=zREQGtSEVj3_FCXiZ5cFR_A@mail.gmail.com>
Date: Wed, 18 Dec 2024 12:00:56 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org, willy@...radead.org, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
	hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com, 
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com, 
	oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, brauner@...nel.org, 
	dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com, 
	lokeshgidra@...gle.com, minchan@...gle.com, jannh@...gle.com, 
	shakeel.butt@...ux.dev, souravpanda@...gle.com, pasha.tatashin@...een.com, 
	klarasmodin@...il.com, corbet@....net, linux-doc@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a
 reference count

On Wed, Dec 18, 2024 at 11:38 AM 'Liam R. Howlett' via kernel-team
<kernel-team@...roid.com> wrote:
>
> * Suren Baghdasaryan <surenb@...gle.com> [241218 14:29]:
> > On Wed, Dec 18, 2024 at 11:07 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > >
> > > On Wed, Dec 18, 2024 at 11:00 AM 'Liam R. Howlett' via kernel-team
> > > <kernel-team@...roid.com> wrote:
> > > >
> > > > * Suren Baghdasaryan <surenb@...gle.com> [241218 12:58]:
> > > > > On Wed, Dec 18, 2024 at 9:44 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > > > > >
> > > > > > On Wed, Dec 18, 2024 at 09:36:42AM -0800, Suren Baghdasaryan wrote:
> > > > > >
> > > > > > > > You will not. vms_complete_munmap_vmas() will call remove_vma() to
> > > > > > > > remove PTEs IIRC, and if you do start_write() and detach() before
> > > > > > > > dropping mmap_lock_write, you should be good.
> > > > > > >
> > > > > > > Ok, I think we will have to move mmap_write_downgrade() inside
> > > > > > > vms_complete_munmap_vmas() to be called after remove_vma().
> > > > > > > vms_clear_ptes() is using vmas, so we can't move remove_vma() before
> > > > > > > mmap_write_downgrade().
> > > > > >
> > > > > > Why ?!
> > > > > >
> > > > > > vms_clear_ptes() and remove_vma() are fine where they are -- there is no
> > > > > > concurrency left at this point.
> > > > > >
> > > > > > Note that by doing vma_start_write() inside vms_complete_munmap_vmas(),
> > > > > > which is *after* the vmas have been unhooked from the mm, you wait for
> > > > > > any concurrent user to go away.
> > > > > >
> > > > > > And since they're unhooked, there can't be any new users.
> > > > > >
> > > > > > So you're the one and only user left, and code is fine the way it is.
> > > > >
> > > > > Ok, let me make sure I understand this part of your proposal. From
> > > > > your earlier email:
> > > > >
> > > > > @@ -1173,6 +1173,11 @@ static void vms_complete_munmap_vmas(struct
> > > > > vma_munmap_struct *vms,
> > > > >         struct vm_area_struct *vma;
> > > > >         struct mm_struct *mm;
> > > > >
> > > > > +       mas_for_each(mas_detach, vma, ULONG_MAX) {
> > > > > +               vma_start_write(next);
> > > > > +               vma_mark_detached(next, true);
> > > > > +       }
> > > > > +
> > > > >         mm = current->mm;
> > > > >         mm->map_count -= vms->vma_count;
> > > > >         mm->locked_vm -= vms->locked_vm;
> > > > >
> > > > > This would mean:
> > > > >
> > > > > vms_complete_munmap_vmas
> > > > >            vma_start_write
> > > > >            vma_mark_detached
> > > > >            mmap_write_downgrade
> > > > >            vms_clear_ptes
> > > > >            remove_vma
> > > > >
> > > > > And remove_vma will be just freeing the vmas. Is that correct?
> > > > > I'm a bit confused because the original thinking was that
> > > > > vma_mark_detached() would drop the last refcnt and if it's 0 we would
> > > > > free the vma right there. If that's still what we want to do then I
> > > > > think the above sequence should look like this:
> > > > >
> > > > > vms_complete_munmap_vmas
> > > > >            vms_clear_ptes
> > > > >            remove_vma
> > > > >                vma_start_write
> > > > >                vma_mark_detached
> > > > >            mmap_write_downgrade
> > > > >
> > > > > because vma_start_write+vma_mark_detached should be done under  mmap_write_lock.
> > > > > Please let me know which way you want to move forward.
> > > > >
> > > >
> > > > Are we sure we're not causing issues with the MAP_FIXED path here?
> > > >
> > > > With the above change, we'd be freeing the PTEs before marking the vmas
> > > > as detached or vma_start_write().
> > >
> > > IIUC when we call vms_complete_munmap_vmas() all vmas inside
> > > mas_detach have been already write-locked, no?
>
> That's the way it is today - but I thought you were moving the lock to
> the complete stage, not adding a new one? (why add a new one otherwise?)

Is my understanding correct that mas_detach is populated by
vms_gather_munmap_vmas() only with vmas that went through
__split_vma() (and were write-locked there)? I don't see any path that
would add any other vma into mas_detach but maybe I'm missing
something?

>
> >
> > Yeah, I think we can simply do this:
> >
> > vms_complete_munmap_vmas
> >            vms_clear_ptes
> >            remove_vma
> >                vma_mark_detached
> >            mmap_write_downgrade
> >
> > If my assumption is incorrect, assertion inside vma_mark_detached()
> > should trigger. I tried a quick test and so far nothing exploded.
> >
>
> If they are write locked, then the page faults are not a concern.  There
> is also the rmap race that Jann found in mmap_region() [1].  This is
> probably also fine since you are keeping the write lock in place earlier
> on in the gather stage.  Note the ptes will already be cleared by the
> time vms_complete_munmap_vmas() is called in this case.
>
> [1] https://lore.kernel.org/all/CAG48ez0ZpGzxi=-5O_uGQ0xKXOmbjeQ0LjZsRJ1Qtf2X5eOr1w@mail.gmail.com/
>
> Thanks,
> Liam
>
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