Re: [Qemu-devel] [PATCH v2 06/21] qcow2: Helper for refcount array reallocation

[Eric Blake <eblake@redhat.com>]

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On 11/14/2014 06:05 AM, Max Reitz wrote:
> Add a helper function for reallocating a refcount array, independently

s/independently/independent/

> of the refcount order. The newly allocated space is zeroed and the
> function handles failed reallocations gracefully.

This patch is doing two things: it is refactoring things into a nice
helper function (mentioned), AND it is adding a guarantee that you now
always allocate a table on cluster boundaries, even when you aren't
using the full table (hinted at elsewhere in the series, but noticeably
absent here).  I think you want to add more comments to the commit
message making that more obvious, since it looks like you rely on that
guarantee later.

> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-refcount.c | 121 +++++++++++++++++++++++++++++--------------------
>  1 file changed, 72 insertions(+), 49 deletions(-)
> 

> +
> +static int realloc_refcount_array(BDRVQcowState *s, uint16_t **array,
> +                                  int64_t *size, int64_t new_size)

I think this function deserves a comment stating that *array is actually
allocated to full cluster size with a 0 tail, so that it can be written
straight to disk.

> +{
> +    /* Round to clusters so the array can be directly written to disk */
> +    size_t old_byte_size = ROUND_UP(refcount_array_byte_size(s, *size),
> +                                    s->cluster_size);
> +    size_t new_byte_size = ROUND_UP(refcount_array_byte_size(s, new_size),
> +                                    s->cluster_size);
> +    uint16_t *new_ptr;

Can old_byte_size ever equal new_byte_size?  Or are we guaranteed that
this will only be called when we really need to add another cluster to
the reftable?

[reading further]

Yes, it looks like *size and new_size are not necessarily
cluster-aligned, so as an example, it is very likely that we might call
realloc_refcount_array with the existing size of 20 and a new size of
21, both of which fit within the same byte size when rounded up to
cluster boundary.  But that means that the realloc is a no-op in that
case; might it be worth special-casing rather than wasting time on the
g_try_realloc and no-op memset?  [at least the code works correctly even
without a special case shortcut]

> +
> +    new_ptr = g_try_realloc(*array, new_byte_size);
> +    if (new_byte_size && !new_ptr) {
> +        return -ENOMEM;
> +    }

Is it worth asserting that new_byte_size is non-zero?  Why would anyone
ever call this to resize down to 0?  (But I can see where you DO call it
with old_byte_size of zero, when initializing data structures and using
this function for the first allocation.)

> +
> +    if (new_ptr) {

If we assert that new_byte_size is non-zero, then at this point, new_ptr
is non-NULL and this condition is pointless.

> +        memset((void *)((uintptr_t)new_ptr + old_byte_size), 0,
> +               new_byte_size - old_byte_size);
> +    }
> +
> +    *array = new_ptr;
> +    *size  = new_size;
> +
> +    return 0;
> +}
>  

Code looks correct as written, whether or not you also add more
comments, asserts, and/or shortcuts for no-op situations.  So:

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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