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Date:   Tue, 28 Jul 2020 15:40:15 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Steven Sistare <steven.sistare@oracle.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Anthony Yznaga <anthony.yznaga@oracle.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-arch@vger.kernel.org, mhocko@kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        hpa@zytor.com, viro@zeniv.linux.org.uk, akpm@linux-foundation.org,
        arnd@arndb.de, keescook@chromium.org, gerg@linux-m68k.org,
        ktkhai@virtuozzo.com, peterz@infradead.org, esyr@redhat.com,
        jgg@ziepe.ca, christian@kellner.me, areber@redhat.com,
        cyphar@cyphar.com, linux-api@vger.kernel.org
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC
Message-ID: <20200728134015.tmjy5hy4xden2v5h@wittgenstein>
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 <28125570-4129-bcba-099b-f90481cfbfe8@oracle.com>
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On Mon, Jul 27, 2020 at 02:00:17PM -0400, Steven Sistare wrote:
> On 7/27/2020 1:07 PM, ebiederm@xmission.com wrote:
> > Anthony Yznaga <anthony.yznaga@oracle.com> writes:
> > 
> >> This patchset adds support for preserving an anonymous memory range across
> >> exec(3) using a new madvise MADV_DOEXEC argument.  The primary benefit for
> >> sharing memory in this manner, as opposed to re-attaching to a named shared
> >> memory segment, is to ensure it is mapped at the same virtual address in
> >> the new process as it was in the old one.  An intended use for this is to
> >> preserve guest memory for guests using vfio while qemu exec's an updated
> >> version of itself.  By ensuring the memory is preserved at a fixed address,
> >> vfio mappings and their associated kernel data structures can remain valid.
> >> In addition, for the qemu use case, qemu instances that back guest RAM with
> >> anonymous memory can be updated.
> > 
> > What is wrong with using a file descriptor to a possibly deleted file
> > and re-mmaping it?
> > 
> > There is already MAP_FIXED that allows you to ensure you have the same
> > address.
> 
> MAP_FIXED blows away any existing mapping in that range, which is not the 
> desired behavior.  We want to preserve the previously created mapping at

There's also MAP_FIXED_NOREPLACE since v4.17 in case that helps.

Note that this should really go to linux-api too. I won't argue to
resend it since that would mean spamming everyone's inbox with the same
thread again but in case you send a revised version, please ensure to Cc
linux-api. The glibc folks are listening on there too.

Thanks!
Christian