Hi,

I'm not qualified to comment on the code, but here are some suggestions on
config option and comments.

Cheers,
FJP

Ryo Tsuruta wrote:
quoted text
> +config DM_BAND
> +	tristate "I/O band width control "

s/band width/bandwidth/
(it seems to be used correctly elsewhere, but you may want to double-check)

quoted text
> +	depends on BLK_DEV_DM
> +	---help---
> +	Any processes or cgroups can use the same storage
> +	with its band-width fairly shared.

s/band-width/bandwidth/

The help should probably be a bit more verbose as this does not tell anybody
much who has not already read the documentation.

Maybe something like:
<snip>
This device-mapper target allows to define how the
available bandwith of a storage device should be
shared between processes or cgroups.

Information on how to use dm-band is available in:
   Documentation/device-mapper/band.txt
</snip>

quoted text
> + * The following functiotons determine when and which BIOs should
                    ^^^^^^^^^^^
quoted text
> + * be submitted to control the I/O flow.
> + * It is possible to add a new I/O scheduling policy with it.

quoted text
> + *  <Method>      <description>
> + * g_can_submit   : To determine whether a given group has a right to

s/a right/the right/

quoted text
> + *                  submit BIOs.
> + * 		    The larger return value the higher priority to submit.
> + * 		    Zero means it has no right.

"The larger the return value, the higher the priority [...]"

quoted text
> + * g_prepare_bio  : Called right before submitting each BIO.
> + * g_restart_bios : Called when there exist some BIOs blocked but none of
> them
> + * 		    can't be submitted now.

s/when there exist some BIOs blocked/if some BIOs exist that are blocked/ ?

"none of them can't" : the double negative looks incorrect (and should be
avoided anyway)

quoted text
> + *		    This method have to do something to restart to submit BIOs.

s/have/has/

"has to do something" : that's rather vague...
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