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From:   Brian Geffon <bgeffon@google.com>
Date:   Mon, 10 Feb 2020 10:38:58 -0800
Message-ID: <CADyq12yJRauv3Xkyv6vVYT7n_Fe6rw-DMw5KJyqG=j_-rzhgUw@mail.gmail.com>
Subject: Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     "Michael S . Tsirkin" <mst@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, linux-api@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>,
        Will Deacon <will@kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Sonny Rao <sonnyrao@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Yu Zhao <yuzhao@google.com>,
        Jesse Barnes <jsbarnes@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Florian Weimer <fweimer@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>
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Thank you Andrew. I'll get working on some self-tests.

Brian

On Sun, Feb 9, 2020 at 5:21 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri,  7 Feb 2020 12:18:56 -0800 Brian Geffon <bgeffon@google.com> wrote:
>
> > When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is
> > set, the source mapping will not be removed. Instead it will be
> > cleared as if a brand new anonymous, private mapping had been created
> > atomically as part of the mremap() call.  If a userfaultfd was watching
> > the source, it will continue to watch the new mapping.  For a mapping
> > that is shared or not anonymous, MREMAP_DONTUNMAP will cause the
> > mremap() call to fail. Because MREMAP_DONTUNMAP always results in moving
> > a VMA you MUST use the MREMAP_MAYMOVE flag. The final result is two
> > equally sized VMAs where the destination contains the PTEs of the source.
> >
> > We hope to use this in Chrome OS where with userfaultfd we could write
> > an anonymous mapping to disk without having to STOP the process or worry
> > about VMA permission changes.
> >
> > This feature also has a use case in Android, Lokesh Gidra has said
> > that "As part of using userfaultfd for GC, We'll have to move the physical
> > pages of the java heap to a separate location. For this purpose mremap
> > will be used. Without the MREMAP_DONTUNMAP flag, when I mremap the java
> > heap, its virtual mapping will be removed as well. Therefore, we'll
> > require performing mmap immediately after. This is not only time consuming
> > but also opens a time window where a native thread may call mmap and
> > reserve the java heap's address range for its own usage. This flag
> > solves the problem."
>
> This seems useful and reasonably mature, so I'll queue it for
> additional testing and shall await review feedback.
>
> Could we please get some self-test code for this feature in
> tools/testing/selftests/vm?  Perhaps in userfaultfd.c?
>