Re: [Xen-devel] xen-blkback unmap with network retansmission will cause a coredump

["Chentao(Boby)" <boby.chen@xxxxxxxxxx>]

On 2014/9/22 18:01, Roger Pau Monné wrote:
> El 20/09/14 a les 12.57, Chentao(Boby) ha escrit:
>> Hi konrad and roger,
>>
>>     When xen-blkback module executes unmap operation, and at the same time 
>> the skb of network retansmission uses this map page, it will cause a crash 
>> of hostos.
>> The crash stack of this problem is like below.
>> <ffffffff8041133e>{do_page_fault+0x38e}
>> <ffffffff8040d9e8>{page_fault+0x28}
>> <ffffffff80223cdb>{memcpy+0xb}
>> <ffffffff802325c2>{swiotlb_tbl_map_single+0x212}
>> <ffffffff8023274a>{swiotlb_map_page+0x17a}
>> <ffffffffa03468e6>{tg3:tg3_start_xmit+0x656}
>> <ffffffff80354d14>{dev_hard_start_xmit+0x334}
>> <ffffffff803721be>{sch_direct_xmit+0x1ae}
>>
>>     I search website, found citrix engineers has met this problem long time 
>> ago. And I realized citrix engineers solve this problem according to modify 
>> kernel stack.
>> Because this modification is very large, linux kernel community hasn't 
>> accept it until now. I have a immature thought, in dispatch_rw_block_io 
>> function, if this io
>> is a write operation, we use grant copy hypercall instead of grant map 
>> hypercall. I verify my modification and it can solve this problem.
>>
>>     What's your opinion of my modification? I am very looking forward to 
>> your reply. Any reply is appreciated.
> 
> Hello,
> 
> Yes, using grant-copy instead of grant-map is going to solve the
> problem, but it also defeats the purpose of persistent grants. I'm
> afraid it is going to introduce a noticeable performance penalty.
> 
Roger, you are right. We found 20%+ performance penalty in 1M 100% sequential 
write 128 depth

when workload is running on ramdisk.

> IMHO a better solution would be to use GNTTABOP_unmap_and_replace with
> the scratch balloon page instead of GNTTABOP_unmap_grant_ref. See
> arch/x86/xen/p2m.c m2p_remove_override for an example implementation of
> this procedure.
>
You mean if we replace GNTTABOP_unmap_grant_ref with GNTTABOP_unmap_and_replace 
in xen-blkback module,

that will solve the problem. Is my understanding right?

> Roger.
> .
> 


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