Re: xen-blkback unmap with network retansmission will cause a coredump

["Roger Pau Monné" <roger.pau@citrix.com>]

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.

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.

Roger.