Re: [Qemu-devel] [PATCH v2] blockdev: acquire aio_context for bitmap add/remove

[Eric Blake <eblake@redhat.com>]

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On 2/12/19 3:37 PM, John Snow wrote:
> 
> 
> On 2/12/19 3:16 PM, Eric Blake wrote:
>> On 2/12/19 2:07 PM, John Snow wrote:
>>> When bitmaps are persistent, they may incur a disk read or write when bitmaps
>>> are added or removed. For configurations like virtio-dataplane, failing to
>>> acquire this lock will abort QEMU when disk IO occurs.
>>>
>>> We used to acquire aio_context as part of the bitmap lookup, so re-introduce
>>> the lock for just the cases that have an IO penalty.
>>
>> It would be nice to call out which commit id dropped the aio_context
>> acquisition during bitmap lookup (making it easier to analyze how long
>> this has broken, and which downstream builds need the backport.
>>
> 
> OK, I will amend this.
> 
> Looks like:
> 
> commit 2119882c7eb7e2c612b24fc0c8d86f5887d6f1c3
> Author: Paolo Bonzini <pbonzini@redhat.com>
> Date:   Mon Jun 5 14:39:03 2017 +0200
> 
> since 2.10.

Hmm. block-dirty-bitmap-add's "persistent":true parameter was also added
in 2.10 in commit fd5ae4cc.  In fact, 2119882c was made at a time when
there were not persistent bitmaps; so I guess that this means we have
always been broken since fd5ae4cc.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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