Re: [PATCH v10 6/9] copy-on-read: skip non-guest reads if no copy needed

[Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>]

On 07.10.2020 13:06, Vladimir Sementsov-Ogievskiy wrote:
> 29.09.2020 15:38, 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>
>> ---
>>   block/copy-on-read.c | 14 ++++++++++----
>>   block/io.c           |  2 +-
>>   2 files changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>> index f53f7e0..5389dca 100644
>> --- a/block/copy-on-read.c
>> +++ b/block/copy-on-read.c
>> @@ -145,10 +145,16 @@ 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) &
> 
> BDRV_REQ_PREFETCH is documented to be only used with 
> BDRV_REQ_COPY_ON_READ. But here
> BDRV_REQ_COPY_ON_READ appears intermediately. We should change 
> documentation in block.h
> in a separate patch (and probably code in bdrv_aligned_preadv())
> 

OK, we will come here without the BDRV_REQ_PREFETCH flag set.
To differ between guest reads and the stream job ones, we would set it 
here by checking for the qiov NULL pointer:


diff --git a/block/copy-on-read.c b/block/copy-on-read.c
index 4e3b1c5..df2c2ab 100644
--- a/block/copy-on-read.c
+++ b/block/copy-on-read.c
@@ -144,6 +144,9 @@ static int coroutine_fn 
cor_co_preadv_part(BlockDriverState *bs,
                                            n, &n);
              if (ret) {
                  local_flags |= BDRV_REQ_COPY_ON_READ;
+                if (!qiov) {
+                    local_flags |= BDRV_REQ_PREFETCH;
+                }
              }
          }

Andrey

>> +            !(local_flags & BDRV_REQ_COPY_ON_READ)) {
>> +            /* Skip non-guest reads if no copy needed */
>> +        } else {
>> +
> 
> extra new-line ?
> 
>> +            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..62b75a5 100644
>> --- a/block/io.c
>> +++ b/block/io.c
>> @@ -1388,7 +1388,7 @@ static int coroutine_fn jk(BdrvChild *child,
>>               qemu_iovec_init_buf(&local_qiov, bounce_buffer, pnum);
>>               ret = bdrv_driver_preadv(bs, cluster_offset, pnum,
>> -                                     &local_qiov, 0, 0);
>> +                                     &local_qiov, 0, flags & 
>> BDRV_REQ_PREFETCH);
> 
> Why? In this place we want to read. We'll write back the data a few 
> lines below. What will we write,
> if underlying driver decide to do nothing because of BDRV_REQ_PREFETCH?
> 

See my comment above please.

>>               if (ret < 0) {
>>                   goto err;
>>               }
>>
> 
>