Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 30 Jul 2020 16:34:50 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     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, ebiederm@xmission.com, 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, steven.sistare@oracle.com
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC
Message-ID: <20200730153450.GH23808@casper.infradead.org>
References: <1595869887-23307-1-git-send-email-anthony.yznaga@oracle.com>
 <20200730152250.GG23808@casper.infradead.org>
 <20200730152705.ol42jppnl4xfhl32@wittgenstein>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200730152705.ol42jppnl4xfhl32@wittgenstein>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Jul 30, 2020 at 05:27:05PM +0200, Christian Brauner wrote:
> On Thu, Jul 30, 2020 at 04:22:50PM +0100, Matthew Wilcox wrote:
> > On Mon, Jul 27, 2020 at 10:11:22AM -0700, Anthony Yznaga wrote:
> > > 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.
> > 
> > I just realised that something else I'm working on might be a suitable
> > alternative to this.  Apologies for not realising it sooner.
> > 
> > http://www.wil.cx/~willy/linux/sileby.html
> 
> Just skimming: make it O_CLOEXEC by default. ;)

I appreciate the suggestion, and it makes sense for many 'return an fd'
interfaces, but the point of mshare() is to, well, share.  So sharing
the fd with a child is a common usecase, unlike say sharing a timerfd.
The only other reason to use mshare() is to pass the fd over a unix
socket to a non-child, and I submit that is far less common than wanting
to share with a child.