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Date:   Sun, 9 Feb 2020 17:21:40 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Brian Geffon <bgeffon@google.com>
Cc:     "Michael S . Tsirkin" <mst@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, linux-kernel@vger.kernel.org,
        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>
Subject: Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().
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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. =A0If a userfaultfd was watching
> the source, it will continue to watch the new mapping. =A0For 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.
>=20
> 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.
>=20
> 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?