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

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Date:   Mon, 2 Oct 2023 11:21:20 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     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,
        lokeshgidra@...gle.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 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.

Thanks,

-- 
Peter Xu