Re: [PATCH v2 1/7] block/block-copy: specialcase first copy_range request

[Andrey Shinkevich]

On 27/11/2019 21:08, Vladimir Sementsov-Ogievskiy wrote:
> In block_copy_do_copy we fallback to read+write if copy_range failed.
> In this case copy_size is larger than defined for buffered IO, and
> there is corresponding commit. Still, backup copies data cluster by
> cluster, and most of requests are limited to one cluster anyway, so the
> only source of this one bad-limited request is copy-before-write
> operation.
> 
> Further patch will move backup to use block_copy directly, than for
> cases where copy_range is not supported, first request will be
> oversized in each backup. It's not good, let's change it now.
> 
> Fix is simple: just limit first copy_range request like buffer-based
> request. If it succeed, set larger copy_range limit.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>   block/block-copy.c | 41 ++++++++++++++++++++++++++++++-----------
>   1 file changed, 30 insertions(+), 11 deletions(-)
> 
> diff --git a/block/block-copy.c b/block/block-copy.c
> index 79798a1567..8602e2cae7 100644
> --- a/block/block-copy.c
> +++ b/block/block-copy.c
> @@ -70,16 +70,19 @@ void block_copy_state_free(BlockCopyState *s)
>       g_free(s);
>   }
>   
> +static uint32_t block_copy_max_transfer(BdrvChild *source, BdrvChild *target)
> +{
> +    return MIN_NON_ZERO(INT_MAX,
> +                        MIN_NON_ZERO(source->bs->bl.max_transfer,
> +                                     target->bs->bl.max_transfer));
> +}
> +
>   BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
>                                        int64_t cluster_size,
>                                        BdrvRequestFlags write_flags, Error 
> **errp)
>   {
>       BlockCopyState *s;
>       BdrvDirtyBitmap *copy_bitmap;
> -    uint32_t max_transfer =
> -            MIN_NON_ZERO(INT_MAX,
> -                         MIN_NON_ZERO(source->bs->bl.max_transfer,
> -                                      target->bs->bl.max_transfer));
>   
>       copy_bitmap = bdrv_create_dirty_bitmap(source->bs, cluster_size, NULL,
>                                              errp);
> @@ -99,7 +102,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, 
> BdrvChild *target,
>           .mem = shres_create(BLOCK_COPY_MAX_MEM),
>       };
>   
> -    if (max_transfer < cluster_size) {
> +    if (block_copy_max_transfer(source, target) < cluster_size) {
>           /*
>            * copy_range does not respect max_transfer. We don't want to bother
>            * with requests smaller than block-copy cluster size, so fallback 
> to
> @@ -114,12 +117,11 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, 
> BdrvChild *target,
>           s->copy_size = cluster_size;
>       } else {
>           /*
> -         * copy_range does not respect max_transfer (it's a TODO), so we 
> factor
> -         * that in here.
> +         * We enable copy-range, but keep small copy_size, until first
> +         * successful copy_range (look at block_copy_do_copy).
>            */
>           s->use_copy_range = true;
> -        s->copy_size = MIN(MAX(cluster_size, BLOCK_COPY_MAX_COPY_RANGE),
> -                           QEMU_ALIGN_DOWN(max_transfer, cluster_size));
> +        s->copy_size = MAX(s->cluster_size, BLOCK_COPY_MAX_BUFFER);
>       }
>   
>       QLIST_INIT(&s->inflight_reqs);
> @@ -168,7 +170,21 @@ static int coroutine_fn 
> block_copy_do_copy(BlockCopyState *s,
>               s->use_copy_range = false;
>               s->copy_size = MAX(s->cluster_size, BLOCK_COPY_MAX_BUFFER);
>               /* Fallback to read+write with allocated buffer */
> -        } else {
> +        } else if (s->use_copy_range) {
> +            /*
> +             * Successful copy-range. Now increase copy_size.
> +             * copy_range does not respect max_transfer (it's a TODO), so we
> +             * factor that in here.
> +             *
> +             * Note: we double-check s->use_copy_range for the case when
> +             * parallel block-copy request unset it during previous
unsets

> +             * bdrv_co_copy_range call.
> +             */
> +            s->copy_size =
> +                    MIN(MAX(s->cluster_size, BLOCK_COPY_MAX_COPY_RANGE),
> +                        QEMU_ALIGN_DOWN(block_copy_max_transfer(s->source,
> +                                                                s->target),
> +                                        s->cluster_size));
>               goto out;
>           }
>       }
> @@ -176,7 +192,10 @@ static int coroutine_fn 
> block_copy_do_copy(BlockCopyState *s,
>       /*
>        * In case of failed copy_range request above, we may proceed with 
> buffered
>        * request larger than BLOCK_COPY_MAX_BUFFER. Still, further requests 
> will
> -     * be properly limited, so don't care too much.
> +     * be properly limited, so don't care too much. Moreover the most 
> possible
> +     * case (copy_range is unsupported for the configuration, so the very 
> first
> +     * copy_range request fails) is handled by setting large copy_size only
> +     * after first successful copy_range.
>        */
>   
>       bounce_buffer = qemu_blockalign(s->source->bs, nbytes);
> 

It would be good to make that clear where the large size of the first 
request comes from and where it is checked with failure.

Reviewed-by: Andrey Shinkevich <address@hidden>
-- 
With the best regards,
Andrey Shinkevich