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

[Andrey Shinkevich]

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@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;
> >       }
> >