[Xen-devel] Re: some questions of IO ring in xenpaging

[Olaf Hering <olaf@xxxxxxxxx>]

On Thu, Sep 01, Olaf Hering wrote:

> On Thu, Sep 01, zhen shi wrote:
> 
> > Hi Olaf --
> > 
> > I have two questions to ask you about xenpaging.
> > 1) When guest os causes page_fault for the accessed page is paging_out
> > or paged, it will execute p2m_mem_paging_populate(). and in
> > p2m_mem_paging_populate() it will first check if the ring is full.
> > when I ran with domU suse11 4G memory and 8vcpus, I found there will
> > be a corruption in checking the ring.
> > For example, if 4vcpus are met with page faults when they access
> > different pages, and there is only four free-requests for the ring.
> > and then they call p2m_mem_paging_populate(),and execute
> > mem_event_check_ring(d) at the same time.All will find ring is not
> > full,and will fill the requests. It will cause the latter request to
> > cover the front request.
> > and I think there should a lock before the mem_event_check_ring(d),
> > and normally it unlock after mem_event_put_request(d, &req).
> > You can review the attached doc of xenpaging_IO_ring.txt to see if my
> > opnion is right.
> 
> Yes, you are right.
> I think mem_event_check_ring() should reserve a reference, and
> mem_event_put_request() should use that reference.
> mem_sharing_alloc_page() even has a comment that this should be done.

Try this patch. It implements some ref counting.

http://lists.xensource.com/archives/html/xen-devel/2011-09/msg00189.html

Olaf

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