Re: [Qemu-devel] [PATCH v3 4/6] nbd: Rely on block layer to break up large requests

[Paolo Bonzini <pbonzini@redhat.com>]

On 15/07/2016 20:32, Eric Blake wrote:
> Now that the block layer will honor max_transfer, we can simplify
> our code to rely on that guarantee.
> 
> The readv code can call directly into nbd-client, just as the
> writev code has done since commit 52a4650.
> 
> Interestingly enough, while qemu-io 'w 0 40m' splits into a 32M
> and 8M transaction, 'w -z 0 40m' splits into two 16M and an 8M,
> because the block layer caps the bounce buffer for writing zeroes
> at 16M.  When we later introduce support for NBD_CMD_WRITE_ZEROES,
> we can get a full 32M zero write (or larger, if the client and
> server negotiate that write zeroes can use a larger size than
> ordinary writes).
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> Reviewed-by: Fam Zheng <famz@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/nbd-client.c | 51 ++++++++-------------------------------------------
>  block/nbd.c        | 12 +++---------
>  2 files changed, 11 insertions(+), 52 deletions(-)
> 
> diff --git a/block/nbd-client.c b/block/nbd-client.c
> index 4cc408d..f1fb58b 100644
> --- a/block/nbd-client.c
> +++ b/block/nbd-client.c
> @@ -217,15 +217,15 @@ static void nbd_coroutine_end(NbdClientSession *s,
>      }
>  }
> 
> -static int nbd_co_readv_1(BlockDriverState *bs, int64_t sector_num,
> -                          int nb_sectors, QEMUIOVector *qiov,
> -                          int offset)
> +int nbd_client_co_readv(BlockDriverState *bs, int64_t sector_num,
> +                        int nb_sectors, QEMUIOVector *qiov)
>  {
>      NbdClientSession *client = nbd_get_client_session(bs);
>      struct nbd_request request = { .type = NBD_CMD_READ };
>      struct nbd_reply reply;
>      ssize_t ret;
> 
> +    assert(nb_sectors <= NBD_MAX_SECTORS);
>      request.from = sector_num * 512;
>      request.len = nb_sectors * 512;
> 
> @@ -234,16 +234,15 @@ static int nbd_co_readv_1(BlockDriverState *bs, int64_t sector_num,
>      if (ret < 0) {
>          reply.error = -ret;
>      } else {
> -        nbd_co_receive_reply(client, &request, &reply, qiov, offset);
> +        nbd_co_receive_reply(client, &request, &reply, qiov, 0);
>      }
>      nbd_coroutine_end(client, &request);
>      return -reply.error;
> 
>  }
> 
> -static int nbd_co_writev_1(BlockDriverState *bs, int64_t sector_num,
> -                           int nb_sectors, QEMUIOVector *qiov,
> -                           int offset, int flags)
> +int nbd_client_co_writev(BlockDriverState *bs, int64_t sector_num,
> +                         int nb_sectors, QEMUIOVector *qiov, int flags)
>  {
>      NbdClientSession *client = nbd_get_client_session(bs);
>      struct nbd_request request = { .type = NBD_CMD_WRITE };
> @@ -255,11 +254,12 @@ static int nbd_co_writev_1(BlockDriverState *bs, int64_t sector_num,
>          request.type |= NBD_CMD_FLAG_FUA;
>      }
> 
> +    assert(nb_sectors <= NBD_MAX_SECTORS);
>      request.from = sector_num * 512;
>      request.len = nb_sectors * 512;
> 
>      nbd_coroutine_start(client, &request);
> -    ret = nbd_co_send_request(bs, &request, qiov, offset);
> +    ret = nbd_co_send_request(bs, &request, qiov, 0);
>      if (ret < 0) {
>          reply.error = -ret;
>      } else {
> @@ -269,41 +269,6 @@ static int nbd_co_writev_1(BlockDriverState *bs, int64_t sector_num,
>      return -reply.error;
>  }
> 
> -int nbd_client_co_readv(BlockDriverState *bs, int64_t sector_num,
> -                        int nb_sectors, QEMUIOVector *qiov)
> -{
> -    int offset = 0;
> -    int ret;
> -    while (nb_sectors > NBD_MAX_SECTORS) {
> -        ret = nbd_co_readv_1(bs, sector_num, NBD_MAX_SECTORS, qiov, offset);
> -        if (ret < 0) {
> -            return ret;
> -        }
> -        offset += NBD_MAX_SECTORS * 512;
> -        sector_num += NBD_MAX_SECTORS;
> -        nb_sectors -= NBD_MAX_SECTORS;
> -    }
> -    return nbd_co_readv_1(bs, sector_num, nb_sectors, qiov, offset);
> -}
> -
> -int nbd_client_co_writev(BlockDriverState *bs, int64_t sector_num,
> -                         int nb_sectors, QEMUIOVector *qiov, int flags)
> -{
> -    int offset = 0;
> -    int ret;
> -    while (nb_sectors > NBD_MAX_SECTORS) {
> -        ret = nbd_co_writev_1(bs, sector_num, NBD_MAX_SECTORS, qiov, offset,
> -                              flags);
> -        if (ret < 0) {
> -            return ret;
> -        }
> -        offset += NBD_MAX_SECTORS * 512;
> -        sector_num += NBD_MAX_SECTORS;
> -        nb_sectors -= NBD_MAX_SECTORS;
> -    }
> -    return nbd_co_writev_1(bs, sector_num, nb_sectors, qiov, offset, flags);
> -}
> -
>  int nbd_client_co_flush(BlockDriverState *bs)
>  {
>      NbdClientSession *client = nbd_get_client_session(bs);
> diff --git a/block/nbd.c b/block/nbd.c
> index 08e5b67..8a13078 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -349,12 +349,6 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
>      return ret;
>  }
> 
> -static int nbd_co_readv(BlockDriverState *bs, int64_t sector_num,
> -                        int nb_sectors, QEMUIOVector *qiov)
> -{
> -    return nbd_client_co_readv(bs, sector_num, nb_sectors, qiov);
> -}
> -
>  static int nbd_co_flush(BlockDriverState *bs)
>  {
>      return nbd_client_co_flush(bs);
> @@ -450,7 +444,7 @@ static BlockDriver bdrv_nbd = {
>      .instance_size              = sizeof(BDRVNBDState),
>      .bdrv_parse_filename        = nbd_parse_filename,
>      .bdrv_file_open             = nbd_open,
> -    .bdrv_co_readv              = nbd_co_readv,
> +    .bdrv_co_readv              = nbd_client_co_readv,
>      .bdrv_co_writev_flags       = nbd_client_co_writev,
>      .bdrv_close                 = nbd_close,
>      .bdrv_co_flush_to_os        = nbd_co_flush,
> @@ -468,7 +462,7 @@ static BlockDriver bdrv_nbd_tcp = {
>      .instance_size              = sizeof(BDRVNBDState),
>      .bdrv_parse_filename        = nbd_parse_filename,
>      .bdrv_file_open             = nbd_open,
> -    .bdrv_co_readv              = nbd_co_readv,
> +    .bdrv_co_readv              = nbd_client_co_readv,
>      .bdrv_co_writev_flags       = nbd_client_co_writev,
>      .bdrv_close                 = nbd_close,
>      .bdrv_co_flush_to_os        = nbd_co_flush,
> @@ -486,7 +480,7 @@ static BlockDriver bdrv_nbd_unix = {
>      .instance_size              = sizeof(BDRVNBDState),
>      .bdrv_parse_filename        = nbd_parse_filename,
>      .bdrv_file_open             = nbd_open,
> -    .bdrv_co_readv              = nbd_co_readv,
> +    .bdrv_co_readv              = nbd_client_co_readv,
>      .bdrv_co_writev_flags       = nbd_client_co_writev,
>      .bdrv_close                 = nbd_close,
>      .bdrv_co_flush_to_os        = nbd_co_flush,
> 

Acked-by: Paolo Bonzini <pbonzini@redhat.com>