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

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Date:   Fri, 9 Apr 2021 15:16:02 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>,
        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 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 4/9/21 2:18 PM, Peter Xu wrote:
> 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 looked pretty closely at the hugetlb specific parts of the minor fault
handling series.  I only took a high level look at the code modifying and
dealing with the userfaultfd API.  The hugetlb specific parts looked fine
to me.  I can take a closer look at the userfaultfd API modifications,
but it would take more time for me to come up to speed on the APIs.
-- 
Mike Kravetz