linux-kernel - Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration

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Date:   Mon, 12 Jun 2023 08:41:37 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, hannes@...xchg.org,
        mhocko@...e.com, josef@...icpanda.com, jack@...e.cz,
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        viro@...iv.linux.org.uk, brauner@...nel.org,
        pasha.tatashin@...een.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration

On Mon, Jun 12, 2023 at 6:28 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Jun 09, 2023 at 03:30:10PM -0700, Suren Baghdasaryan wrote:
> > On Fri, Jun 9, 2023 at 1:42 PM Peter Xu <peterx@...hat.com> wrote:
> > >
> > > On Thu, Jun 08, 2023 at 05:51:56PM -0700, Suren Baghdasaryan wrote:
> > > > migration_entry_wait does not need VMA lock, therefore it can be dropped
> > > > before waiting. Introduce VM_FAULT_VMA_UNLOCKED to indicate that VMA
> > > > lock was dropped while in handle_mm_fault().
> > > > Note that once VMA lock is dropped, the VMA reference can't be used as
> > > > there are no guarantees it was not freed.
> > >
> > > Then vma lock behaves differently from mmap read lock, am I right?  Can we
> > > still make them match on behaviors, or there's reason not to do so?
> >
> > I think we could match their behavior by also dropping mmap_lock here
> > when fault is handled under mmap_lock (!(fault->flags &
> > FAULT_FLAG_VMA_LOCK)).
> > I missed the fact that VM_FAULT_COMPLETED can be used to skip dropping
> > mmap_lock in do_page_fault(), so indeed, I might be able to use
> > VM_FAULT_COMPLETED to skip vma_end_read(vma) for per-vma locks as well
> > instead of introducing FAULT_FLAG_VMA_LOCK. I think that was your idea
> > of reusing existing flags?
>
> Yes.
>
> I'd suggest we move this patch out of the series as it's not really part of
> it on enabling swap + uffd.  It can be a separate patch and hopefully it'll
> always change both vma+mmap lock cases, and with proper reasonings.

Ok, I can move it out with mmap_lock support only and then add per-vma
lock support in my patchset (because this path is still part of
do_swap_page and my patchset enables swap support for per-vma locks).

>
> Thanks,
>
> --
> Peter Xu
>