Re: Fatal crash on xen4.2 HVM + qemu-xen dm + NFS

[Alex Bligh <alex@alex.org.uk>]

Stefano,

--On 16 January 2013 14:34:34 +0000 Stefano Stabellini 
<stefano.stabellini@eu.citrix.com> wrote:

> It seems that the grant mapping is already gone by the time
> tcp_retransmit is called.
> That might happen because QEMU already completed the read/write
> operation and called xc_gnttab_munmap, that causes the grant_table and
> the m2p_override to remove the p2m and m2p mappings of the foreign
> pages.

What I want to know is why QEMU is completing the read/write operation
before the write (as it surely must be a write) has completed in any
case. This /seems/ to happen only if a backing file is being used
but I'm not sure if that's just triggering the retransmits due to
(e.g.) a slow filer.

If QEMU is completing writes before they've actually been done, haven't
we got a wider set of problems to worry about?

Could the problem be "cache=writeback" on the QEMU command
line (evident from a 'ps'). If caching is writeback perhaps QEMU
needs to copy the data. Is there some setting to turn this off in
xl for test purposes?

> Isn't there a way to prevent tcp_retransmit from running when the
> request is already completed? Or stop it if you find out that the pages
> are already gone?

But what would you do? If you don't run the tcp_retransmit the write
would be lost (to say nothing of the NFS connection to the server).

> You could try persistent grants, that wouldn't solve the bug but they
> should be able to "hide" it pretty well. Not ideal, I know.
> The QEMU side commit is 9e496d7458bb01b717afe22db10a724db57d53fd.
> Konrad issued a pull request recently with the corresponding Linux
> blkfront changes:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git
> stable/for-jens-3.8

That's presumably the fir 8 commits at:
http://git.kernel.org/?p=linux/kernel/git/konrad/xen.git;a=shortlog;h=refs/heads/stable/for-jens-3.8

So I'd need a new dom0 kernel and to backport the QEMU patch.

-- 
Alex Bligh