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From:   Daniel Colascione <dancol@google.com>
Date:   Tue, 11 Feb 2020 15:13:50 -0800
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Subject: Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().
To:     Brian Geffon <bgeffon@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Linux API <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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On Fri, Feb 7, 2020 at 12:19 PM 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.

The left-behind mapping (the "as if a brand new anonymous, private
mapping" map) is immediately writable, right? If so, we need to
account the additional commit charge. What about making the
left-behind mapping PROT_NONE? This way, we'll still solve the
address-space race in Lokesh's use case (because even a PROT_NONE
mapping reserves address space) but won't incur any additional commit
until someone calls mprotect(PROT_WRITE) on the left-behind mapping.
Imagine having two equal-sized mappings and wanting to use mremap() to
swap them: you can implement this swap by carving off a third region
of address space and making two mremap() calls. But without the
PROT_NONE, you pay additional commit for that third region even if you
don't need it.