linux-kernel - Re: [PATCH v4] mm: Add MREMAP

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Message-ID: <CAKOZuevHH1pamEKy5n5RLWDP=tHk6_9bR+g3G+HKnqm_srHvrw@mail.gmail.com>
Date:   Tue, 11 Feb 2020 15:13:50 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Deacon <will@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().

On Fri, Feb 7, 2020 at 12:19 PM Brian Geffon <bgeffon@...gle.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.