linux-kernel - Re: [PATCH v4 03/10] userfaultfd/shmem: support UFFDIO

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Message-ID: <CANsGZ6YRucupVi8Hhu=4Y7yhoY+eFr0YVahLSXpSC2-AikX4qQ@mail.gmail.com>
Date:   Tue, 27 Apr 2021 13:59:12 -0700
From:   Hugh Dickins <hughd@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Shaohua Li <shli@...com>, Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Wang Qing <wangqing@...o.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Brian Geffon <bgeffon@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v4 03/10] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem

On Tue, Apr 27, 2021 at 1:42 PM Peter Xu <peterx@...hat.com> wrote:
> On Tue, Apr 27, 2021 at 01:29:14PM -0700, Axel Rasmussen wrote:
> > On Tue, Apr 27, 2021 at 11:03 AM Peter Xu <peterx@...hat.com> wrote:
> > >
> > > On Tue, Apr 27, 2021 at 09:57:16AM -0700, Axel Rasmussen wrote:
> > > > I'd prefer to keep them separate, as they are not tiny patches (they
> > > > are roughly +200/-150 each). And, they really are quite independent -
> > > > at least in the sense that I can reorder them via rebase with no
> > > > conflicts, and the code builds at each commit in either orientation. I
> > > > think this implies they're easier to review separately, rather than
> > > > squashed.
> > > >
> > > > I don't have a strong feeling about the order. I slightly prefer
> > > > swapping them compared to this v4 series: first introduce minor
> > > > faults, then introduce CONTINUE.
> > > >
> > > > Since Peter also has no strong opinion, and Hugh it sounds like you
> > > > prefer it the other way around, I'll swap them as we had in some
> > > > previous version of this series: first introduce minor faults, then
> > > > introduce CONTINUE.
> > >
> > > Yes I have no strong opinion, but that's probably the least I prefer. :-)
> > >
> > > Because you'll declare UFFD_FEATURE_MINOR_SHMEM and enable this feature without
> > > the feature being completely implemented (without UFFDIO_CONTINUE, it's not
> > > complete since no one will be able to resolve that minor fault).
> > >
> > > Not a big deal anyway, but since we're at it... Basically I think three things
> > > to do for minor shmem support:
> > >
> > >   (1) UFFDIO_CONTINUE (resolving path)
> > >   (2) Handle fault path for shmem minor fault (faulting path)
> > >   (3) Enablement of UFFD_FEATURE_MINOR_SHMEM (from which point, user can detect
> > >       and enable it)
> > >
> > > I have no preference on how you'd like to merge these steps (right now you did
> > > 1 first, then 2+3 later; or as Hugh suggested do 1+2+3 together), but I'd still
> > > hope item 3 should always be the last, if possible...
> >
> > In that case, I'll split the patch which adds the faulting path in
> > two: add the faulting path hook and registration mode, and then in a
> > separate commit advertise the feature flag as available.
> >
> > Then I'll order them like so, which I think is the order Hugh finds
> > more natural:
> > 1. MInor fault registration / faulting path
> > 2. CONTINUE ioctl to resolve the faults
> > 3. Advertise the feature as supported
> >
> > Sound okay?
>
> Good to me, thanks Axel.

Okay.