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

[Bob Liu <bob.liu@xxxxxxxxxx>]

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.

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.

> But in general networked block storage isn't safe without this patch.
> 

That's true!

Thanks,
-Bob

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