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

[Max Reitz <mreitz@redhat.com>]

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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.

> +            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


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