Re: [Qemu-devel] [PATCH v3 06/19] block: make bdrv_co_do_write_zeroes stricter in producing aligned requests

[Peter Lieven <pl@kamp.de>]

On 22.11.2013 13:39, Paolo Bonzini wrote:
> Right now, bdrv_co_do_write_zeroes will only try to align the
> beginning of the request.  However, it is simpler for many
> formats to expect the block layer to separate both the head *and*
> the tail.  This makes sure that the format's bdrv_co_write_zeroes
> function will be called with aligned sector_num and nb_sectors for
> the bulk of the request.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   block.c | 35 +++++++++++++++++++++++------------
>   1 file changed, 23 insertions(+), 12 deletions(-)
>
> diff --git a/block.c b/block.c
> index b18ee6b..de66b21 100644
> --- a/block.c
> +++ b/block.c
> @@ -2768,14 +2768,21 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>       while (nb_sectors > 0 && !ret) {
>           int num = nb_sectors;
>   
> -        /* align request */
> -        if (bs->bl.write_zeroes_alignment &&
> -            num >= bs->bl.write_zeroes_alignment &&
> -            sector_num % bs->bl.write_zeroes_alignment) {
> -            if (num > bs->bl.write_zeroes_alignment) {
> +        /* Align request.  Block drivers can expect the "bulk" of the request
> +         * to be aligned.
> +         */
> +        if (bs->bl.write_zeroes_alignment
> +            && num > bs->bl.write_zeroes_alignment) {
> +            if (sector_num % bs->bl.write_zeroes_alignment != 0) {
> +                /* Make a small request up to the first aligned sector.  */
>                   num = bs->bl.write_zeroes_alignment;
> +                num -= sector_num % bs->bl.write_zeroes_alignment;
> +            } else if ((sector_num + num) % bs->bl.write_zeroes_alignment != 0) {
> +                /* Shorten the request to the last aligned sector.  num cannot
> +                 * underflow because num > bs->bl.write_zeroes_alignment.
> +                 */
> +                num -= (sector_num + num) % bs->bl.write_zeroes_alignment;
>               }
> -            num -= sector_num % bs->bl.write_zeroes_alignment;
>           }
>   
>           /* limit request size */
> @@ -2793,16 +2800,20 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>               /* Fall back to bounce buffer if write zeroes is unsupported */
>               iov.iov_len = num * BDRV_SECTOR_SIZE;
>               if (iov.iov_base == NULL) {
> -                /* allocate bounce buffer only once and ensure that it
> -                 * is big enough for this and all future requests.
> -                 */
> -                size_t bufsize = num <= nb_sectors ? num : max_write_zeroes;
> -                iov.iov_base = qemu_blockalign(bs, bufsize * BDRV_SECTOR_SIZE);
> -                memset(iov.iov_base, 0, bufsize * BDRV_SECTOR_SIZE);
> +                iov.iov_base = qemu_blockalign(bs, num * BDRV_SECTOR_SIZE);
> +                memset(iov.iov_base, 0, num * BDRV_SECTOR_SIZE);
>               }
>               qemu_iovec_init_external(&qiov, &iov, 1);
>   
>               ret = drv->bdrv_co_writev(bs, sector_num, num, &qiov);
> +
> +            /* Keep bounce buffer around if it is big enough for all
> +             * all future requests.
> +             */
> +            if (num < max_write_zeroes) {
> +                qemu_vfree(iov.iov_base);
> +                iov.iov_base = NULL;
> +            }
>           }
>   
>           sector_num += num;
Reviewed-by: Peter Lieven <pl@kamp.de>