Re: [Qemu-devel] [PATCH v2 01/15] block: introduce BDRV_REQ_ALLOCATE flag

[Eric Blake]

On 06/01/2017 10:14 AM, 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 has to return -ENOTSUP if allocation cannot be done efficiently
> (i.e. without falling back to writing actual buffers)
> 
> Signed-off-by: Anton Nefedov <address@hidden>
> ---
>  block/io.c            | 19 ++++++++++++++++---
>  block/trace-events    |  1 +
>  include/block/block.h |  6 +++++-
>  3 files changed, 22 insertions(+), 4 deletions(-)

You may want to 'git config diff.orderFile /path/to/file' (with a
suitably populated file) so that .h files come first in your diffs, as
that can aid reviewers.  At one point, there was a thread about adding
such a file to qemu.git proper for everyone to share, although it seems
to have stalled.

> 
> diff --git a/block/io.c b/block/io.c
> index ed31810..d47efa9 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1272,7 +1272,7 @@ 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)) {

I'd feel MUCH better if you first fixed the conditional just above this
point to ensure that if the caller requests BDRV_REQ_ALLOCATE that we do
not call bdrv->bdrv_co_pwrite_zeroes() unless bs->supported_zero_flags
also mentions this bit.

Remember, the existing semantics of .bdrv_co_pwrite_zeroes() merely
state that we must return -ENOTSUP unless we can guarantee that we read
back as zeroes, but puts no timing constraints on it.  A driver that has
not been retrofitted to understand the BDRV_REQ_ALLOCATE flag will
therefore risk taking too long.  Using bs->supported_zero_flags as your
gate is what will let you avoid calling into a driver that has not been
audited for fitting the new contract.

>              /* Fall back to bounce buffer if write zeroes is unsupported */
>              BdrvRequestFlags write_flags = flags & ~BDRV_REQ_ZERO_WRITE;
>  
> @@ -1355,8 +1355,8 @@ 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)) {
>          flags |= BDRV_REQ_ZERO_WRITE;
>          if (bs->detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP) {
>              flags |= BDRV_REQ_MAY_UNMAP;
> @@ -1436,6 +1436,9 @@ 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;
> +        }

Can we assert that BDRV_REQ_ALLOCATE will only be supplied by a caller
that is already using aligned values?  Or is that too strict?

>          buf = qemu_blockalign(bs, align);
>          iov = (struct iovec) {
>              .iov_base   = buf,
> @@ -1534,6 +1537,11 @@ int coroutine_fn bdrv_co_pwritev(BdrvChild *child,
>          return ret;
>      }
>  
> +    if (qiov && flags & BDRV_REQ_ALLOCATE) {
> +        /* allocation request with qiov provided doesn't make much sense */
> +        return -ENOTSUP;

Should this be an assertion (bug in the program for mixing things that
don't make sense) rather than just a runtime error return?

> +    }
> +
>      bdrv_inc_in_flight(bs);
>      /*
>       * Align write if necessary by performing a read-modify-write cycle.
> @@ -1665,6 +1673,11 @@ int coroutine_fn bdrv_co_pwrite_zeroes(BdrvChild 
> *child, int64_t offset,
>  {
>      trace_bdrv_co_pwrite_zeroes(child->bs, offset, count, flags);
>  
> +    if (flags & BDRV_REQ_MAY_UNMAP && flags & BDRV_REQ_ALLOCATE) {
> +        /* nonsense */
> +        return -ENOTSUP;
> +    }

Ditto.

> +
>      if (!(child->bs->open_flags & BDRV_O_UNMAP)) {
>          flags &= ~BDRV_REQ_MAY_UNMAP;
>      }
> diff --git a/block/trace-events b/block/trace-events
> index 9a71c7f..a15c2cc 100644
> --- a/block/trace-events
> +++ b/block/trace-events
> @@ -15,6 +15,7 @@ bdrv_aio_writev(void *bs, int64_t sector_num, int 
> nb_sectors, void *opaque) "bs
>  bdrv_co_readv(void *bs, int64_t sector_num, int nb_sector) "bs %p sector_num 
> %"PRId64" nb_sectors %d"
>  bdrv_co_writev(void *bs, int64_t sector_num, int nb_sector) "bs %p 
> sector_num %"PRId64" nb_sectors %d"
>  bdrv_co_pwrite_zeroes(void *bs, int64_t offset, int count, int flags) "bs %p 
> offset %"PRId64" count %d flags %#x"
> +bdrv_co_allocate(void *bs, int64_t offset, int count) "bs %p offset 
> %"PRId64" count %d"
>  bdrv_co_do_copy_on_readv(void *bs, int64_t offset, unsigned int bytes, 
> int64_t cluster_offset, unsigned int cluster_bytes) "bs %p offset %"PRId64" 
> bytes %u cluster_offset %"PRId64" cluster_bytes %u"
>  
>  # block/stream.c
> diff --git a/include/block/block.h b/include/block/block.h
> index 9b355e9..53a357c 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,
> +    /* BDRV_REQ_ALLOCATE is used to indicate that the driver is to
> +     * efficiently allocate the space so it reads as zeroes or return an 
> error
> +     */
> +    BDRV_REQ_ALLOCATE           = 0x40,

Doesn't match how the other flags are documented, but any documentation
is better than none.

Missing mention of the new flag in the documentation for
supported_zero_flags.

>  
>      /* Mask of valid flags */
> -    BDRV_REQ_MASK               = 0x3f,
> +    BDRV_REQ_MASK               = 0x7f,
>  } BdrvRequestFlags;
>  
>  typedef struct BlockSizes {
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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