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Re: [Xen-devel] xen-blkfront hang

On 08/04/2017 05:13 PM, Roger Pau Monné wrote:
> On Fri, Aug 04, 2017 at 10:00:09AM +0200, Valentin Vidic wrote:
>> On Mon, Jul 31, 2017 at 09:09:19AM +0800, Dongli Zhang wrote:
>>> To verify whether the above patch would help, please check the 
>>> nr_grant_frames
>>> value in guest domU. If this value is exactly the same of maximum grant 
>>> frames
>>> (by default, xen mainline uses 32) and the number of free grant references 
>>> is
>>> very small, the above patch might help.
>> You are right, this is what I get after the machine hangs:
>>   crash> print nr_grant_frames 
>>   $1 = 32
>>   crash> print gnttab_free_count 
>>   $2 = 9
>>> The best way is to increase the gnttab_max_frames to larger value (e.g.,  
>>> 256)
>>> in dom0 xen.gz grub.
>> Thank you, this seems to help.  The test machine does not hang now and
>> the numbers are looking better now:
>>   crash> print nr_grant_frames 
>>   $1 = 59
>>   crash> print gnttab_free_count 
>>   $2 = 356
> At some point I've already expressed my opinion that having a
> per-queue list of persistent grants was not a good idea. Now I think
> the only solution is to remove persistent grants, or to lower the
> default number of per-queue persistent grants to a ridiculously low
> value.

It would be more efficient if (1) persistent grants are shared by all queues and
(2) there is a new mechanism to allow frontend to actively ask backend to unmap
existing persistent grants. So far, it is backend's responsibility to decide
when to unmap persistent grants.

Dongli Zhang

> Roger.

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