Re: [PATCH v11 09/13] copy-on-read: skip non-guest reads if no copy needed - Vladimir Sementsov-Ogievskiy

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
To: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>,
	Max Reitz <mreitz@redhat.com>,
	qemu-block@nongnu.org
Cc: qemu-devel@nongnu.org, kwolf@redhat.com, fam@euphon.net,
	stefanha@redhat.com, armbru@redhat.com, jsnow@redhat.com,
	libvir-list@redhat.com, eblake@redhat.com, den@openvz.org
Subject: Re: [PATCH v11 09/13] copy-on-read: skip non-guest reads if no copy needed
Date: Thu, 22 Oct 2020 11:56:41 +0300	[thread overview]
Message-ID: <84e40ccd-a3a6-4676-d514-3535d1017c0f@virtuozzo.com> (raw)
In-Reply-To: <4ae47559-af39-1dcc-5e22-f7259b55dfee@virtuozzo.com>

22.10.2020 10:50, Andrey Shinkevich wrote:
> 
> On 21.10.2020 23:43, Andrey Shinkevich wrote:
>> On 14.10.2020 18:22, Vladimir Sementsov-Ogievskiy wrote:
>>> 14.10.2020 15:51, Max Reitz wrote:
>>>> On 12.10.20 19:43, Andrey Shinkevich wrote:
>>>>> If the flag BDRV_REQ_PREFETCH was set, pass it further to the
>>>>> COR-driver to skip unneeded reading. It can be taken into account for
>>>>> the COR-algorithms optimization. That check is being made during the
>>>>> block stream job by the moment.
>>>>>
>>>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>>>> ---
> 
> [...]
> 
>>>>> diff --git a/block/io.c b/block/io.c
>>>>> index 11df188..bff1808 100644
>>>>> --- a/block/io.c
>>>>> +++ b/block/io.c
>>>>> @@ -1512,7 +1512,8 @@ static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
>>>>>       max_bytes = ROUND_UP(MAX(0, total_bytes - offset), align);
>>>>>       if (bytes <= max_bytes && bytes <= max_transfer) {
>>>>> -        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset, 0);
>>>>> +        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset,
>>>>> +                                 flags & bs->supported_read_flags);
>>>
>>>
>>> When BDRV_REQ_PREFETCH is passed, qiov may be (and generally should be) NULL. This means, that we can't just drop the flag when call the driver that doesn't support it.
>>>
>>> Actually, if driver doesn't support the PREFETCH flag we should do nothing.
>>>
>>>
>>>>
>>>> Ah, OK.  I see.  I expected this to be a separate patch.  I still wonder
>>>> why it isn’t.
>>>>
>>>
>>>
>>> Could it be part of patch 07? I mean introduce new field supported_read_flags and handle it in generic code in one patch, prior to implementing support for it in COR driver.
>>>
>>>
>>
>> We have to add the supported flags for the COR driver in the same patch. Or before handling the supported_read_flags at the generic layer (handling zero does not make a sence). Otherwise, the test #216 (where the COR-filter is applied) will not pass.
>>
>> Andrey
> 
> I have found a workaround and am going to send all the related patches as a separate series.
> 

What is the problem?

If in a separate patch prior to modifying COR driver, we add new field supported_read_flags and add a support for it in generic layer, when no driver support it yet, nothing should be changed and all tests should pass..

-- 
Best regards,
Vladimir