Re: [Qemu-devel] [PATCH 1/2] qemu-img: make sure contain the consecutive number of zero bytes

[Eric Blake]

On 04/23/2017 09:33 AM, address@hidden wrote:
> From: Lidong Chen <address@hidden>
> 
> is_allocated_sectors_min don't guarantee to contain the
> consecutive number of zero bytes. this patch fixes this bug.

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helpful.

> 
> Signed-off-by: Lidong Chen <address@hidden>
> ---
>  qemu-img.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index b220cf7..df6d165 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -1060,9 +1060,9 @@ static int is_allocated_sectors(const uint8_t *buf, int 
> n, int *pnum)
>  }
>  
>  /*
> - * Like is_allocated_sectors, but if the buffer starts with a used sector,
> - * up to 'min' consecutive sectors containing zeros are ignored. This avoids
> - * breaking up write requests for only small sparse areas.
> + * Like is_allocated_sectors, but up to 'min' consecutive sectors
> + * containing zeros are ignored. This avoids breaking up write requests
> + * for only small sparse areas.
>   */
>  static int is_allocated_sectors_min(const uint8_t *buf, int n, int *pnum,
>      int min)
> @@ -1071,11 +1071,12 @@ static int is_allocated_sectors_min(const uint8_t 
> *buf, int n, int *pnum,
>      int num_checked, num_used;
>  
>      if (n < min) {
> -        min = n;
> +        *pnum = n;
> +        return 1;
>      }
>  
>      ret = is_allocated_sectors(buf, n, pnum);
> -    if (!ret) {
> +    if (!ret && *pnum >= min) {

I seem to recall past attempts to try and patch this function, which
were then turned down, although I haven't scrubbed the archives for a
quick URL to point to. I'm worried that there are more subtleties here
than what you realize.

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

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