xen-blkback unmap with network retansmission will cause a coredump

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

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.

    Best wishes.

    Tao Chen