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

[Andrey Shinkevich]

On 14.10.2020 14:59, 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)
> >   {
> > -    return bdrv_co_preadv_part(bs->file, offset, bytes, qiov, qiov_offset,
> > -                               flags | BDRV_REQ_COPY_ON_READ);
> > +    int64_t n = 0;
> > +    int64_t size = offset + bytes;
> > +    int local_flags;
> > +    int ret;
> > +    BDRVStateCOR *state = bs->opaque;
> > +
> > +    if (!state->base_overlay) {
> > +        return bdrv_co_preadv_part(bs->file, offset, bytes, qiov, qiov_offset,
> > +                                   flags | BDRV_REQ_COPY_ON_READ);
> > +    }
> > +
> > +    while (offset < size) {
> > +        local_flags = flags;
> > +
> > +        /* In case of failure, try to copy-on-read anyway */
> > +        ret = bdrv_is_allocated(bs->file->bs, offset, bytes, &n);
> > +        if (!ret) {
>
> In case of failure, a negative value is going to be returned, we won’t
> go into this conditional block, and local_flags isn’t going to contain
> BDRV_REQ_COPY_ON_READ.
>
> So the idea of CORing in case of failure sounds sound to me, but it
> doesn’t look like that’s done.

Yes, it's obvious. That was just my fault to miss setting the additional 
condition for "ret < 0". Thank you for noticing that.

Andrey

> > +            ret = bdrv_is_allocated_above(bdrv_cow_bs(bs->file->bs),
>
> I think this should either be bdrv_backing_chain_next() or we must rule
> out the possibility of bs->file->bs being a filter somewhere.  I think
> I’d prefer the former.
>
> > +                                          state->base_overlay, true, offset,
> > +                                          n, &n);
> > +            if (ret) {
>
> “ret == 1 || ret < 0” would be more explicit (and in line with the “!ret
> || ret < 0” probably needed above), but correct either way.
>
> Max