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

[zhen shi <bickys1986@xxxxxxxxx>]

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.

2)mem_sharing and xenpaging are shared with one IO ring for domU.In
the function of mem_sharing_alloc_page(),if alloc_domheap_page(d, 0)
returns NULL,then it will pause VCPU ,check if the ring is full,and
fill the request at last.
I think there is also a corruption of mem_event_check_ring(d) with it
in p2m_mem_paging_populate().We should assure
exclusively in reading the free_request and puting requests.
What's more,although it hardly fails in alloc_domheap_page(d, 0) from
mem_sharing_alloc_page() ,it will fill the requests in IO ring.
But  in xenpaging when handling the page_in requests,we have not
distinguished the requests with flag "MEM_EVENT_FLAG_VCPU_PAUSED" from
paging or sharing.It will cause if the request is from
mem_sharing_alloc_page(),it will
go to p2m_mem_paging_resume() at last,and the page's p2mt is
p2m_ram_rw.I think this is wrong.Maybe we should add the req.type when
page in .

I'm so sorry to have a poor English.But I look forward to your early reply.


                                                              Thank
you！

Attachment: xenpaging_IO_ring.txt
Description: Text document

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