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Date: Sun, 29 Jan 2017 14:03:29 +0100
From: Greg Kurz <groug@kaod.org>
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Subject: Re: [Qemu-devel] [PATCH] migrate: Migration aborts abruptly for
 machine "none"
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To: Ashijeet Acharya <ashijeetacharya@gmail.com>
Cc: Thomas Huth <thuth@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, "dgilbert@redhat.com" <dgilbert@redhat.com>, Juan Quintela <quintela@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

On Sun, 29 Jan 2017 01:06:47 +0530
Ashijeet Acharya <ashijeetacharya@gmail.com> wrote:

> On Sun, Jan 29, 2017 at 12:11 AM, Greg Kurz <groug@kaod.org> wrote:
> > On Thu, 26 Jan 2017 14:46:52 +0530
> > Ashijeet Acharya <ashijeetacharya@gmail.com> wrote:
> >  
> >> Migration of a "none" machine with no RAM crashes abruptly as
> >> bitmap_new() fails and thus aborts. Instead, place a check for
> >> last_ram_offset() being '0' at the start of ram_save_setup() and
> >> error out with a meaningful error message.
> >>
> >> Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>
> >> ---  
> >  
> 
> cc'ing Paolo in : I had an IRC chat with him and he has a very
> interesting twist in the tale to add here.
> 
> > Maybe a naive question: why a "none" machine with zero RAM should fail to
> > migrate ?  
> 
> Assuming you are referring to why its failing ATM; it fails because

My question was more: why deciding to fail migration instead of fixing the
crash ? One would naively think that no RAM is *just* less state to
migrate... but maybe the current code assumes that a machine always has
RAM.

> g_try_malloc0() inside bitmap_try_new() returns a NULL pointer for
> zero bits and thus the check for NULL inside bitmap_new() becomes true
> and it aborts. Check bitmap_new() for convenience.
> 
> Ignore the noise if you already knew this! :-)
> 

I hadn't checked, thanks for the details.

Cheers.

--
Greg

> Ashijeet