Re: [Qemu-devel] [PATCH for-2.12] qcow2: Reset free_cluster_index when allocating a new refcount block

[Eric Blake]

On 03/20/2018 08:55 AM, Alberto Garcia wrote:
> When we try to allocate new clusters we first look for available ones
> starting from s->free_cluster_index and once we find them we increase
> their reference counts. Before we get to call update_refcount() to do
> this last step s->free_cluster_index is already pointing to the next
> cluster after the ones we are trying to allocate.
> During update_refcount() it may happen however that we also need to
> allocate a new refcount block in order to store the refcounts of these
> new clusters

Your changes to test 121 covers this...

> (and to complicate things further that may also require
> us to grow the refcount table).

...but not this.  Is it worth also trying to cover this case in the 
testsuite as well?  Probably made easier if you use a refcount_order of 
6 instead of the default of 4, so that it is easier to make the refcount 
table spill into a second cluster because refcount blocks have fewer 
entries.

> This can be reproduced easily:
>
>       qemu-img create -f qcow2 -o cluster_size=512 hd.qcow2 1M
>       qemu-io -c 'write 0 124k' hd.qcow2
>
> After this the image has 132608 bytes (256 clusters), and the refcount
> block is full. If we write 512 more bytes it should allocate two new
> clusters: the data cluster itself and a new refcount block.
>
>       qemu-io -c 'write 124k 512' hd.qcow2

Nice test case!  And yes, 512-byte clusters are easier for proving when 
we have inefficient allocation strategies.

> What this patch does is reset s->free_cluster_index to its previous
> value when alloc_refcount_block() returns -EAGAIN. This way the caller
> will try to allocate again the original clusters if they are still
> free.
>
> The output of iotest 026 also needs to be updated because now that
> images have no holes some tests fail at a different point and the
> number of leaked clusters is different.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>   block/qcow2-refcount.c     |  7 +++++++
>   tests/qemu-iotests/026.out |  6 +++---
>   tests/qemu-iotests/121     | 20 ++++++++++++++++++++
>   tests/qemu-iotests/121.out | 10 ++++++++++
>   4 files changed, 40 insertions(+), 3 deletions(-)

Long-standing bug, but it IS a bug fix, so I think it is safe for 2.12.

> +            /* If the caller needs to restart the search for free clusters,
> +             * try the same ones first to see if they're still free. */
> +            if (ret == -EAGAIN) {
> +                if (s->free_cluster_index > (start >> s->cluster_bits)) {
> +                    s->free_cluster_index = (start >> s->cluster_bits);
> +                }

Is there any harm in making this assignment unconditional, instead of 
only doing it when free_cluster_index has grown larger than start?  [And 
unrelated, but it might be nice to do a followup cleanup to track 
free_cluster_offset by bytes instead of having to shift 
free_cluster_index everywhere]

At any rate, my comments can be deferred to later if desired, so

Reviewed-by: Eric Blake <address@hidden>

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