[Libguestfs] [PATCH nbdkit v5 FINAL 14/19] data, memory: Implement extents.

[Eric Blake]

On 3/28/19 11:18 AM, Richard W.M. Jones wrote:
> These plugins are both based on the same sparse array structure which
> supports a simple implementation of extents.
> ---
>  common/sparse/sparse.h     |   7 +-
>  common/sparse/sparse.c     |  37 ++++++++++-
>  plugins/data/data.c        |  16 ++++-
>  plugins/memory/memory.c    |  16 ++++-
>  README                     |   2 +
>  tests/Makefile.am          |   2 +
>  tests/test-data-extents.sh | 131 +++++++++++++++++++++++++++++++++++++
>  7 files changed, 207 insertions(+), 4 deletions(-)
> 
> +int
> +sparse_array_extents (struct sparse_array *sa,
> +                      uint32_t count, uint64_t offset,
> +                      struct nbdkit_extents *extents)
> +{
> +  uint32_t n, type;
> +  void *p;
> +
> +  while (count > 0) {
> +    p = lookup (sa, offset, false, &n, NULL);
> +    if (n > count)
> +      n = count;

I know in earlier review I asked whether we can move the clamping...

> +
> +    /* Work out the type of this extent. */
> +    if (p == NULL)
> +      /* No backing page, so it's a hole. */
> +      type = NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO;
> +    else {
> +      if (is_zero (p, n))
> +        /* A backing page and it's all zero, it's a zero extent. */
> +        type = NBDKIT_EXTENT_ZERO;
> +      else
> +        /* Normal allocated data. */
> +        type = 0;
> +    }
> +    if (nbdkit_add_extent (extents, offset, n, type) == -1)
> +      return -1;
> +

...to here, after the final nbdkit_add_extent, so that we can return a
larger extent than the client's request. I remember when I originally
asked, you declined due to odd interactions with REQ_ONE semantics, but
since then, we changed how add_extent() works. Does it work now to defer
the clamping?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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