Re: [PATCH] migration, xen: Fix block image lock issue on live migration

[Anthony PERARD <anthony.perard@citrix.com>]

On Wed, Nov 29, 2017 at 12:28:39PM +0000, Julien Grall wrote:
> + Stefano
> 
> On 11/27/2017 03:00 PM, Anthony PERARD wrote:
> > Hi Julien,
> 
> Hi Anthony,
> 
> > 
> > Can I get a release-ack for this patch?
> > 
> > This fix local live migration of HVM guest when the disk backend is
> > qdisk.  osstest doesn't report a regression because the kernel or the
> > glibc is just a bit too old.
> 
> When does that regression happen? I am considering to release Xen 4.10 soon
> and would need more details to decide the inclusion of the patch.

That can happen when an API call Open File Description Locks
(F_OFD_GETLK and F_OFD_SETLK) is available. There are since Linux 3.15
and glibc 2.20.

When this API is available, QEMU is going to set a lock on the file open
for a block device. When we try to live migrate a guest and the lock is
set, the new QEMU is going to fail to open the disk with an error
message that say something like "file locked", and migration fail.

The scenario when the lock is an issue are:
- an HVM guest which us qdisk for disk backend. (Like when the disk
  image is in a format different than raw. e.g. qcow2)
- an HVM guest which only use emulated disk.


BUT I can't reproduce the issue with the QEMU we are going to release...
so there is probably a race involve. I guess the patch can wait.

-- 
Anthony PERARD

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