Re: [Xen-devel] [PATCHv5 12/14] xen-blkback: safely unmap grants in case they are still in use

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 09/03/15 10:51, Bob Liu wrote:
> 
> On 03/09/2015 05:30 PM, David Vrabel wrote:
>> On 09/03/15 09:09, Bob Liu wrote:
>>> Hi David,
>>>
>>> Recently I met an issue which is likely related with this patch. It
>>> happened when running block benchmark on domU, the backend was an iSCSI
>>> disk connected to dom0. I got below panic at put_page_testzero() on
>>> dom0, at that time the ixgbe network card was freeing skb pages in
>>> __skb_frag_unref() but the page->_count was already 0.
>>> Do you think is it possiable that page was already freed by blkback?
>>
>> It's possible, but in this case I think the blkback device must have
>> been destroyed for this to have happened,  because blkback doesn't free
>> the pages until it is destroyed.
>>
> 
> Sorry, I didn't get the point here, doesn't bio_complete free pages?
> E.g.
> __end_block_io_op() > xen_blkbk_unmap_and_respond() > put_free_pages()
> Then shrink_free_pagepool() free the page finally.

Ug. That's all the persistent grant stuff I'm not very familiar with.
Perhaps Roger can comment?

> I'm worry about there may be race condition some place and caused this
> panic, but it's hard to locate. It may exist in xen-blkback/grant or
> even the ixgbe network driver itself.

Actually, freeing the page in blkback early shouldn't cause this sort of
thing anyway, since the skb should hold additional refs to the pages.

David

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