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

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Message-ID: <CAJHvVciMU=TDGxArtEQSq3n5DCLfYNWh7bVX_8dQL_dht4Q73w@mail.gmail.com>
Date:   Tue, 27 Apr 2021 13:29:14 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Hugh Dickins <hughd@...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@...r.kernel.org,
        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 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?

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