Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Paolo Bonzini <pbonzini@redhat.com>]

Il 18/03/2013 14:49, Alex Bligh ha scritto:
> 
> 
> --On 18 March 2013 14:32:23 +0100 Paolo Bonzini <pbonzini@redhat.com>
> wrote:
> 
>> How does migration work with this change?  The target may read stale
>> data from the page cache.  Fix the kernel bug please.
> 
> Presumably the same way as if writeback caching is selected. I presume
> that must fsync() / fdatasync() all the data to disk, and a barrier will
> produce one of those.

No, that's done already.  The source does an fsync/fdatasync before
terminating the migration.

The problem is that the target's page cache might host image data from a
previous run.  If you do not use O_DIRECT, it will not see the changes
made on the source.

> It would be great to fix the kernel bug (and I have submitted code), but
> the fix is pretty intrusive (see the link I posted) and there appears
> to be little interest in taking it forward. Certainly my kernel hacking
> skills are not adequate to the task.
> 
> The current position is that booting a Xen domU which does disk I/O
> (Ubuntu cloud image used as the test case) with an NFS root crashes dom0
> absolutely repeatably, and kills all other guests. Unless and until
> there is a kernel fix for that, Xen is in essence unusable with HVM
> and network based disk backend. So we need a workaround in the meantime
> which doesn't require a kernel fix.

If you want to have this patch, you need to detect the bug and only do
the hack if the bug is detect.  Plus, disable migration when the hack is
in use.

Also:

1) why does blkback not have the bug?

2) does it also affect virtio disks (or perhaps AHCI too)?  I think
Stefano experimented with virtio in Xen.  If it does, then you're
working around the problem in the wrong place.

Paolo