linux-kernel - Re: [RFC PATCH 0/5] madvise MADV

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Date:   Tue, 28 Jul 2020 15:40:15 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Steven Sistare <steven.sistare@...cle.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org, mhocko@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        arnd@...db.de, keescook@...omium.org, gerg@...ux-m68k.org,
        ktkhai@...tuozzo.com, peterz@...radead.org, esyr@...hat.com,
        jgg@...pe.ca, christian@...lner.me, areber@...hat.com,
        cyphar@...har.com, linux-api@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

On Mon, Jul 27, 2020 at 02:00:17PM -0400, Steven Sistare wrote:
> On 7/27/2020 1:07 PM, ebiederm@...ssion.com wrote:
> > Anthony Yznaga <anthony.yznaga@...cle.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