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Re: [Qemu-devel] [PATCH v6 4/9] qcow2: make refcount size calculation co


From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v6 4/9] qcow2: make refcount size calculation conservative
Date: Mon, 8 May 2017 10:00:45 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0

On 05/08/2017 09:15 AM, Stefan Hajnoczi wrote:
> The refcount metadata size calculation is inaccurate and can produce
> numbers that are too small.  This is bad because we should calculate a
> conservative number - one that is guaranteed to be large enough.
> 
> This patch switches the approach to a fixed point calculation because
> the existing equation is hard to solve when inaccuracies are taken care
> of.
> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> Reviewed-by: Alberto Garcia <address@hidden>
> ---
>  block/qcow2.c | 82 
> ++++++++++++++++++++++++++++++-----------------------------
>  1 file changed, 42 insertions(+), 40 deletions(-)
> 

> -    nrefblocke = (aligned_total_size + meta_size + cluster_size)
> -        / (cluster_size - rces - rces * sizeof(uint64_t)
> -                / cluster_size);
> -    meta_size += DIV_ROUND_UP(nrefblocke, refblock_size) * cluster_size;
> -
> -    /* total size of refcount tables */
> -    nreftablee = nrefblocke / refblock_size;
> -    nreftablee = align_offset(nreftablee, cluster_size / sizeof(uint64_t));
> -    meta_size += nreftablee * sizeof(uint64_t);
> +    /* total size of refcount table and blocks */
> +    meta_size += qcow2_refcount_metadata_size(
> +            (meta_size + aligned_total_size) / cluster_size,

How does this interact with Max's patch which avoids truncating division
in favor of rounding up?
https://lists.gnu.org/archive/html/qemu-devel/2017-05/msg00690.html

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

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