Re: [PATCH v11 09/13] copy-on-read: skip non-guest reads if no copy needed
On 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(a)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 &?
Yes, that's correct.
> + !(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:
I played with the flags to make the idea obvious for the eye of a
beholder: "we neither read nor write".
Comparing the BDRV_REQ_PREFETCH against the 'flags' means that the flag
comes from outside of the function.
And the empty section means we do nothing in that case.
Eventually, I will pick up the brief expression below.
Thanks,
Andrey
((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);
Ah, OK. I see. I expected this to be a separate patch. I still wonder
why it isn’t.
Max
> goto out;
> }
>
>