Re: [PATCH v2] btrfs-progs: fi resize: fix false 0.00B sized output

[Boris Burkov <boris@bur.io>]

On Mon, Apr 19, 2021 at 09:05:49PM +0800, Su Yue wrote:
> Resize to nums without sign prefix makes false output:
>  btrfs fi resize 1:150g /srv/extra
> Resize device id 1 (/dev/sdb1) from 298.09GiB to 0.00B
> 
> The resize operation would take effect though.
> 
> Fix it by handling the case if mod is 0 in check_resize_args().
> 
> Issue: #307
> Reported-by: Chris Murphy <lists@colorremedies.com>
> Signed-off-by: Su Yue <l@damenly.su>
> ---
>  cmds/filesystem.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> ---
> Changelog:
>     v2:
>         Calculate u64 diff using max() and min().
>         Calculate mod by comparing new and old size.
> diff --git a/cmds/filesystem.c b/cmds/filesystem.c
> index 9e3cce687d6e..54d46470c53f 100644
> --- a/cmds/filesystem.c
> +++ b/cmds/filesystem.c
> @@ -1158,6 +1158,16 @@ static int check_resize_args(const char *amount, const char *path) {
>  		}
>  		old_size = di_args[dev_idx].total_bytes;
>  
> +		/* For target sizes without '+'/'-' sign prefix(e.g. 1:150g) */
> +		if (mod == 0) {
> +			new_size = diff;
> +			diff = max(old_size, new_size) - min(old_size, new_size);
> +			if (new_size > old_size)
> +				mod = 1;
> +			else if (new_size < old_size)
> +				mod = -1;
> +		}
> +
>  		if (mod < 0) {
>  			if (diff > old_size) {
>  				error("current size is %s which is smaller than %s",

This fix seems correct to me, but it feels a tiny bit over-complicated.
Personally, I think it would be cleaner to do something like:

if (mod == 0) {
	new_size = diff;
} else if (mod < 0) {
	// >0 check
	new_size = old_size - diff
} else {
	// overflow check
	new_size = old_size + diff
}

At this point, new_size is set correctly in all cases and we can do the
print. I tested this version on some simple cases, and it seems to work
ok as well.

Thanks for the fix,
Boris

> -- 
> 2.30.1
>