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

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

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())

> +            !(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?

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


-- 
Best regards,
Vladimir