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

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Date:   Tue, 4 Aug 2020 12:13:16 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     'James Bottomley' <James.Bottomley@...senpartnership.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Steven Sistare <steven.sistare@...cle.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
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        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
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        "areber@...hat.com" <areber@...hat.com>,
        "cyphar@...har.com" <cyphar@...har.com>
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

On Tue, Aug 04, 2020 at 08:44:42AM +0000, David Laight wrote:
> From: James Bottomley
> > Sent: 03 August 2020 16:43
> > 
> > On Mon, 2020-08-03 at 10:28 -0500, Eric W. Biederman wrote:
> > [...]
> > > What is wrong with live migration between one qemu process and
> > > another qemu process on the same machine not work for this use case?
> > >
> > > Just reusing live migration would seem to be the simplest path of
> > > all, as the code is already implemented.  Further if something goes
> > > wrong with the live migration you can fallback to the existing
> > > process.  With exec there is no fallback if the new version does not
> > > properly support the handoff protocol of the old version.
> > 
> > Actually, could I ask this another way: the other patch set you sent to
> > the KVM list was to snapshot the VM to a PKRAM capsule preserved across
> > kexec using zero copy for extremely fast save/restore.  The original
> > idea was to use this as part of a CRIU based snapshot, kexec to new
> > system, restore.  However, why can't you do a local snapshot, restart
> > qemu, restore using the PKRAM capsule to achieve exactly the same as
> > MADV_DOEXEC does but using a system that's easy to reason about?  It
> > may be slightly slower, but I think we're still talking milliseconds.
> 
> 
> I've had another idea (that is probably impossible...).
> What about a 'reverse mmap' operation.
> Something that creates an fd whose contents are a chunk of the
> processes address space.

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