Re: [PATCH v11 05/13] copy-on-read: limit COR operations to base in COR driver

[Andrey Shinkevich]

On 14.10.2020 15:01, Max Reitz wrote:
> On 12.10.20 19:43, Andrey Shinkevich wrote:
> > Limit COR operations by the base node in the backing chain when the
> > overlay base node name is given. It will be useful for a block stream
> > job when the COR-filter is applied. The overlay base node is passed as
> > the base itself may change due to concurrent commit jobs on the same
> > backing chain.
> >
> > Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
> > ---
> >   block/copy-on-read.c | 39 +++++++++++++++++++++++++++++++++++++--
> >   1 file changed, 37 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/copy-on-read.c b/block/copy-on-read.c
> > index c578b1b..dfbd6ad 100644
> > --- a/block/copy-on-read.c
> > +++ b/block/copy-on-read.c
> > @@ -122,8 +122,43 @@ static int coroutine_fn 
> > cor_co_preadv_part(BlockDriverState *bs,
> >                                              size_t qiov_offset,
> >                                              int flags)
> >   {

[...]

> > +            ret = bdrv_is_allocated_above(bdrv_cow_bs(bs->file->bs),
> > +                                          state->base_overlay, true, offset,
> > +                                          n, &n);
> > +            if (ret) {
> > +                local_flags |= BDRV_REQ_COPY_ON_READ;
> > +            }
> > +        }
>
> Furthermore, I just noticed – can the is_allocated functions not return
> 0 in @n, when @offset is a the EOF?  Is that something to look out for?
>   (I’m not sure.)
>
> Max

The check for EOF is managed earlier in the stream_run() for a 
block-stream job. For other cases of using the COR-filter, the check for 
EOF can be added to the cor_co_preadv_part().
I would be more than happy if we can escape the duplicated checking for 
is_allocated in the block-stream. But how the stream_run() can stop 
calling the blk_co_preadv() when EOF is reached if is_allocated removed 
from it? May the cor_co_preadv_part() return EOF (or other error code) 
to be handled by a caller if (ret == 0 && n == 0 && (flags & 
BDRV_REQ_PREFETCH)?

Andrey