linux-kernel - Re: [PATCH v5 6/6] mm: handle userfaults under VMA lock

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Message-ID: <CAJuCfpER=0GzcR3sWETYJAPK9SKAaRYtNfVXa-sXZu8MiL67NA@mail.gmail.com>
Date:   Thu, 29 Jun 2023 09:39:41 -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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        michel@...pinasse.org, liam.howlett@...cle.com, jglisse@...gle.com,
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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 v5 6/6] mm: handle userfaults under VMA lock

On Thu, Jun 29, 2023 at 9:33 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Wed, Jun 28, 2023 at 05:19:31PM -0700, Suren Baghdasaryan wrote:
> > On Wed, Jun 28, 2023 at 10:32 AM Peter Xu <peterx@...hat.com> wrote:
> > >
> > > On Wed, Jun 28, 2023 at 10:25:29AM -0700, Suren Baghdasaryan wrote:
> > > > Enable handle_userfault to operate under VMA lock by releasing VMA lock
> > > > instead of mmap_lock and retrying. Note that FAULT_FLAG_RETRY_NOWAIT
> > > > should never be used when handling faults under per-VMA lock protection
> > > > because that would break the assumption that lock is dropped on retry.
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > >
> > > Maybe the sanitize_fault_flags() changes suite more in patch 3, but not a
> > > big deal I guess.
> >
> > IIUC FAULT_FLAG_RETRY_NOWAIT comes into play in this patchset only in
> > the context of uffds, therefore that check seems to be needed when we
> > enable per-VMA lock uffd support, which is this patch. Does that make
> > sense?
>
> I don't see why uffd is special in this regard, as e.g. swap also checks
> NOWAIT when folio_lock_or_retry() so I assume it's also used there.
>
> IMHO the "NOWAIT should never apply with VMA_LOCK so far" assumption starts
> from patch 3 where it conditionally releases the vma lock when
> !(RETRY|COMPLETE); that is the real place where it can start to go wrong if
> anyone breaks the assumption.

Um, yes, you are right as usual. It was clear to me from the code that
NOWAIT is not used with swap under VMA_LOCK, that's why I didn't
consider this check earlier. Yeah, patch 3 seems like a more
appropriate place for it. I'll move it and post a new patchset later
today or tomorrow morning with your Acks.
Thanks,
Suren.

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