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

[Andrey Shinkevich]

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;
> >               }