linux-kernel - Re: [PATCH v2 2/3] userfaultfd: UFFDIO

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Date:   Mon, 2 Oct 2023 16:55:35 +0100
From:   Lokesh Gidra <lokeshgidra@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>, Jann Horn <jannh@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...hat.com,
        hughd@...gle.com, mhocko@...e.com, axelrasmussen@...gle.com,
        rppt@...nel.org, willy@...radead.org, Liam.Howlett@...cle.com,
        zhangpeng362@...wei.com, bgeffon@...gle.com,
        kaleshsingh@...gle.com, ngeoffray@...gle.com, jdduke@...gle.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

On Mon, Oct 2, 2023 at 4:46 PM Lokesh Gidra <lokeshgidra@...gle.com> wrote:
>
> On Mon, Oct 2, 2023 at 4:21 PM Peter Xu <peterx@...hat.com> wrote:
> >
> > On Mon, Oct 02, 2023 at 10:00:03AM +0200, David Hildenbrand wrote:
> > > In case we cannot simply remap the page, the fallback sequence (from the
> > > cover letter) would be triggered.
> > >
> > > 1) UFFDIO_COPY
> > > 2) MADV_DONTNEED
> > >
> > > So we would just handle the operation internally without a fallback.
> >
> > Note that I think there will be a slight difference on whole remap
> > atomicity, on what happens if the page is modified after UFFDIO_COPY but
> > before DONTNEED.
> >
> > UFFDIO_REMAP guarantees full atomicity when moving the page, IOW, threads
> > can be updating the pages when ioctl(UFFDIO_REMAP), data won't get lost
> > during movement, and it will generate a missing event after moved, with
> > latest data showing up on dest.
> >
> > I'm not sure that means such a fallback is a problem, Suren may know
> > better with the use case.
>
> Although there is no problem in using fallback with our use case but
> as a user of userfaultfd, I'd suggest leaving it to the developer.
> Failing with appropriate errno makes more sense. If handled in the
> kernel, then the user may assume at the end of the operation that the
> src vma is completely unmapped. And if not correctness issues, it
> could lead to memory leaks.

I meant that in addition to the possibility of correctness issues due
to lack of atomicity, it could also lead to memory leaks, as the user
may assume that src vma is empty post-operation. IMHO, it's better to
fail with errno so that the user would fix the code with necessary
changes (like using DONTFORK, if forking).
> >
> > Thanks,
> >
> > --
> > Peter Xu
> >