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

[Vladimir Sementsov-Ogievskiy]

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 at virtuozzo.com>
>> ---
>>   block/copy-on-read.c | 13 +++++++++----
>>   block/io.c           |  3 ++-
>>   2 files changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>> index b136895..278a11a 100644
>> --- a/block/copy-on-read.c
>> +++ b/block/copy-on-read.c
>> @@ -148,10 +148,15 @@ static int coroutine_fn cor_co_preadv_part(BlockDriverState *bs,
>>               }
>>           }
>>   
>> -        ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
>> -                                  local_flags);
>> -        if (ret < 0) {
>> -            return ret;
>> +        if (!!(flags & BDRV_REQ_PREFETCH) &
> 
> How about dropping the double negation and using a logical && instead of
> the binary &?
> 
>> +            !(local_flags & BDRV_REQ_COPY_ON_READ)) {
>> +            /* Skip non-guest reads if no copy needed */
>> +        } else {
> 
> Hm.  I would have just written the negated form
> 
> (!(flags & BDRV_REQ_PREFETCH) || (local_flags & BDRV_REQ_COPY_ON_READ))
> 
> and put the “skip” comment above that condition.
> 
> (Since local_flags is initialized to flags, it can be written as a
> single comparison, but that’s a matter of taste and I’m not going to
> recommend either over the other:
> 
> ((local_flags & (BDRV_REQ_PREFETCH | BDRV_REQ_COPY_ON_READ)) !=
> BDRV_REQ_PREFETCH)
> 
> )
> 
>> +            ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
>> +                                      local_flags);
>> +            if (ret < 0) {
>> +                return ret;
>> +            }
>>           }
>>   
>>           offset += n;
>> 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.


-- 
Best regards,
Vladimir