linux-kernel - Re: [PATCH 0/9] userfaultfd: add minor fault handling for shmem

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Date:   Fri, 9 Apr 2021 17:18:24 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Daniel Colascione <dancol@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        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 0/9] userfaultfd: add minor fault handling for shmem

On Fri, Apr 09, 2021 at 10:03:53AM -0700, Axel Rasmussen wrote:
> On Thu, Apr 8, 2021 at 10:04 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > On Thu,  8 Apr 2021 16:43:18 -0700 Axel Rasmussen <axelrasmussen@...gle.com> wrote:
> >
> > > The idea is that it will apply cleanly to akpm's tree, *replacing* the following
> > > patches (i.e., drop these first, and then apply this series):
> > >
> > > userfaultfd-support-minor-fault-handling-for-shmem.patch
> > > userfaultfd-support-minor-fault-handling-for-shmem-fix.patch
> > > userfaultfd-support-minor-fault-handling-for-shmem-fix-2.patch
> > > userfaultfd-support-minor-fault-handling-for-shmem-fix-3.patch
> > > userfaultfd-support-minor-fault-handling-for-shmem-fix-4.patch
> > > userfaultfd-selftests-use-memfd_create-for-shmem-test-type.patch
> > > userfaultfd-selftests-create-alias-mappings-in-the-shmem-test.patch
> > > userfaultfd-selftests-reinitialize-test-context-in-each-test.patch
> > > userfaultfd-selftests-exercise-minor-fault-handling-shmem-support.patch
> >
> > Well.  the problem is,
> >
> > > +     if (area_alias == MAP_FAILED)
> > > +             err("mmap of memfd alias failed");
> >
> > `err' doesn't exist until eleventy patches later, in Peter's
> > "userfaultfd/selftests: unify error handling".  I got tired of (and
> > lost confidence in) replacing "err(...)" with "fprintf(stderr, ...);
> > exit(1)" everywhere then fixing up the fallout when Peter's patch came
> > along.  Shudder.
> 
> Oof - sorry about that!
> 
> >
> > Sorry, all this material pretty clearly isn't going to make 5.12
> > (potentially nine days hence), so I shall drop all the userfaultfd
> > patches.  Let's take a fresh run at all of this after -rc1.
> 
> That's okay, my understanding was already that it certainly wouldn't
> be in the 5.12 release, but that we might be ready in time for 5.13.
> 
> >
> >
> > I have tentatively retained the first series:
> >
> > userfaultfd-add-minor-fault-registration-mode.patch
> > userfaultfd-add-minor-fault-registration-mode-fix.patch
> > userfaultfd-disable-huge-pmd-sharing-for-minor-registered-vmas.patch
> > userfaultfd-hugetlbfs-only-compile-uffd-helpers-if-config-enabled.patch
> > userfaultfd-add-uffdio_continue-ioctl.patch
> > userfaultfd-update-documentation-to-describe-minor-fault-handling.patch
> > userfaultfd-selftests-add-test-exercising-minor-fault-handling.patch
> >
> > but I don't believe they have had much testing standalone, without the
> > other userfaultfd patches present.  So I don't think it's smart to
> > upstream these in this cycle.  Or I could drop them so you and Peter
> > can have a clean shot at redoing the whole thing.  Please let me know.
> 
> From my perspective, both Peter's error handling and the hugetlbfs
> minor faulting patches are ready to go. (Peter's most importantly; we
> should establish that as a base, and put all the burden on resolving
> conflicts with it on us instead of you :).)
> 
> My memory was that Peter's patch was applied before my shmem series,
> but it seems I was mistaken. So, maybe the best thing to do is to have
> Peter send a version of it based on your tree, without the shmem
> series? And then I'll resolve any conflicts in my tree?
> 
> It's true that we haven't tested the hugetlbfs minor faults patch
> extensively *with the shmem one also applied*, but it has had more
> thorough review than the shmem one at this point (e.g. by Mike
> Kravetz), and they're rather separate code paths (I'd be surprised if
> one breaks the other).

Yes I think the hugetlb part should have got more review done.  IMHO it's a
matter of whether Mike would still like to do a more thorough review, or seems
okay to keep them.

I can repost the selftest series later if needed, as long as I figured which is
the suitable base commit.  Those selftest patches are definitely not urgent for
this release, so we can wait for the next release.

Thanks,

-- 
Peter Xu