Re: [PATCH v4 03/10] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem

From: Axel Rasmussen <axelrasmussen@google.com>
To: Peter Xu <peterx@redhat.com>
Cc: Hugh Dickins <hughd@google.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jerome Glisse <jglisse@redhat.com>, Joe Perches <joe@perches.com>,
	Lokesh Gidra <lokeshgidra@google.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>, Shaohua Li <shli@fb.com>,
	Shuah Khan <shuah@kernel.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Wang Qing <wangqing@vivo.com>,
	linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	LKML <linux-kernel@vger.kernel.org>,
	linux-kselftest@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
	Brian Geffon <bgeffon@google.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Mina Almasry <almasrymina@google.com>,
	Oliver Upton <oupton@google.com>
Subject: Re: [PATCH v4 03/10] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem
Date: Tue, 27 Apr 2021 13:29:14 -0700	[thread overview]
Message-ID: <CAJHvVciMU=TDGxArtEQSq3n5DCLfYNWh7bVX_8dQL_dht4Q73w@mail.gmail.com> (raw)
In-Reply-To: <20210427180314.GD6820@xz-x1>

On Tue, Apr 27, 2021 at 11:03 AM Peter Xu <peterx@redhat.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
>