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

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Date:   Tue, 27 Apr 2021 14:03:14 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.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 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...

Thanks,

-- 
Peter Xu