Re: [Qemu-devel] [PATCH v7 3/9] block: introduce BDRV_REQ_ALLOCATE flag

[Max Reitz <mreitz@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 5325 bytes --]

On 2018-01-18 18:49, Anton Nefedov wrote:
> The flag is supposed to indicate that the region of the disk image has
> to be sufficiently allocated so it reads as zeroes.
> 
> The call with the flag set must return -ENOTSUP if allocation cannot
> be done efficiently.
> This has to be made sure of by both
>   - the drivers that support the flag
>   - and the common block layer (so it will not fall back to any slowpath
>     (like writing zero buffers) in case the driver does not support
>     the flag).
> 
> Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> Reviewed-by: Alberto Garcia <berto@igalia.com>
> ---
>  include/block/block.h     |  6 +++++-
>  include/block/block_int.h |  2 +-
>  block/io.c                | 20 +++++++++++++++++---
>  3 files changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/include/block/block.h b/include/block/block.h
> index 9b12774..3e31b89 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -65,9 +65,13 @@ typedef enum {
>      BDRV_REQ_NO_SERIALISING     = 0x8,
>      BDRV_REQ_FUA                = 0x10,
>      BDRV_REQ_WRITE_COMPRESSED   = 0x20,
> +    /* The BDRV_REQ_ALLOCATE flag is used to indicate that the driver has to
> +     * efficiently allocate the space so it reads as zeroes, or return an error.

What happens if you specify this for a normal write operation that does
not write zeroes?

(I suppose the answer is "don't do that", but that would need to be
documented more clearly here.)

> +     */
> +    BDRV_REQ_ALLOCATE           = 0x40,
>  
>      /* Mask of valid flags */
> -    BDRV_REQ_MASK               = 0x3f,
> +    BDRV_REQ_MASK               = 0x7f,
>  } BdrvRequestFlags;
>  
>  typedef struct BlockSizes {
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 29cafa4..b141710 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -632,7 +632,7 @@ struct BlockDriverState {
>      /* Flags honored during pwrite (so far: BDRV_REQ_FUA) */
>      unsigned int supported_write_flags;
>      /* Flags honored during pwrite_zeroes (so far: BDRV_REQ_FUA,
> -     * BDRV_REQ_MAY_UNMAP) */
> +     * BDRV_REQ_MAY_UNMAP, BDRV_REQ_ALLOCATE) */
>      unsigned int supported_zero_flags;
>  
>      /* the following member gives a name to every node on the bs graph. */
> diff --git a/block/io.c b/block/io.c
> index 7ea4023..cf2f84c 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1424,7 +1424,7 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
>              assert(!bs->supported_zero_flags);
>          }
>  
> -        if (ret == -ENOTSUP) {
> +        if (ret == -ENOTSUP && !(flags & BDRV_REQ_ALLOCATE)) {
>              /* Fall back to bounce buffer if write zeroes is unsupported */
>              BdrvRequestFlags write_flags = flags & ~BDRV_REQ_ZERO_WRITE;
>  
> @@ -1514,8 +1514,8 @@ static int coroutine_fn bdrv_aligned_pwritev(BdrvChild *child,
>      ret = notifier_with_return_list_notify(&bs->before_write_notifiers, req);
>  
>      if (!ret && bs->detect_zeroes != BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF &&
> -        !(flags & BDRV_REQ_ZERO_WRITE) && drv->bdrv_co_pwrite_zeroes &&
> -        qemu_iovec_is_zero(qiov)) {
> +        !(flags & BDRV_REQ_ZERO_WRITE) && !(flags & BDRV_REQ_ALLOCATE) &&
> +        drv->bdrv_co_pwrite_zeroes && qemu_iovec_is_zero(qiov)) {

Do we really need to add the BDRV_REQ_ALLOCATE check here?  If the
caller specifies that flag, then we won't invalidate it by adding the
BDRV_REQ_ZERO_WRITE flag (as long as we don't add BDRV_REQ_MAY_UNMAP).

>          flags |= BDRV_REQ_ZERO_WRITE;
>          if (bs->detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP) {
>              flags |= BDRV_REQ_MAY_UNMAP;
> @@ -1593,6 +1593,9 @@ static int coroutine_fn bdrv_co_do_zero_pwritev(BdrvChild *child,
>  
>      assert(flags & BDRV_REQ_ZERO_WRITE);
>      if (head_padding_bytes || tail_padding_bytes) {
> +        if (flags & BDRV_REQ_ALLOCATE) {
> +            return -ENOTSUP;
> +        }
>          buf = qemu_blockalign(bs, align);
>          iov = (struct iovec) {
>              .iov_base   = buf,
> @@ -1693,6 +1696,9 @@ int coroutine_fn bdrv_co_pwritev(BdrvChild *child,
>          return ret;
>      }
>  
> +    /* allocation request with qiov provided doesn't make much sense */
> +    assert(!(qiov && (flags & BDRV_REQ_ALLOCATE)));
> +

So I suppose the use of BDRV_REQ_ALLOCATE necessitates the use of
BDRV_REQ_ZERO_WRITE?  That should be documented, then.

Max

>      bdrv_inc_in_flight(bs);
>      /*
>       * Align write if necessary by performing a read-modify-write cycle.
> @@ -1822,6 +1828,14 @@ int coroutine_fn bdrv_co_pwrite_zeroes(BdrvChild *child, int64_t offset,
>  {
>      trace_bdrv_co_pwrite_zeroes(child->bs, offset, bytes, flags);
>  
> +    assert(!((flags & BDRV_REQ_MAY_UNMAP) && (flags & BDRV_REQ_ALLOCATE)));
> +
> +    if ((flags & BDRV_REQ_ALLOCATE) &&
> +        !(child->bs->supported_zero_flags & BDRV_REQ_ALLOCATE))
> +    {
> +        return -ENOTSUP;
> +    }
> +
>      if (!(child->bs->open_flags & BDRV_O_UNMAP)) {
>          flags &= ~BDRV_REQ_MAY_UNMAP;
>      }
> 



[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 512 bytes --]