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

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Date:   Mon, 03 Aug 2020 08:42:50 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Steven Sistare <steven.sistare@...cle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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, christian.brauner@...ntu.com,
        peterz@...radead.org, esyr@...hat.com, jgg@...pe.ca,
        christian@...lner.me, areber@...hat.com, cyphar@...har.com
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

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.

James