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

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Message-ID: <ZIcddoWjYlDXNKJA@x1n>
Date:   Mon, 12 Jun 2023 09:28:22 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.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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Subject: Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration

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.

Thanks,

-- 
Peter Xu