[dm-devel] [PATCH 0/2] Fix blkdev_issue_discard()

[Mike Snitzer]

On Tue, Dec 15 2015 at  9:38pm -0500,
Martin K. Petersen <martin.petersen at oracle.com> wrote:

> >>>>> "Bart" == Bart Van Assche <bart.vanassche at sandisk.com> writes:
> 
> Bart> Should the caller of blkdev_issue_discard() implement this or
> Bart> should this functionality be added in blkdev_issue_discard()
> Bart> itself ?
> 
> I'm very much against dropping information in the ioctl/filesystem
> submission path. But I am not against having a helper function of some
> sort that DM and target could use to handle heads and tails.
> 
> Right now DM does not handle this and I think it would be worthwhile to
> have. Mike, what are your requirements?

If the discard doesn't conform to the device's stacked discard limits
then yes the head and/or tail will get dropped on the floor.

Not sure why DM is being made the focal point on this.  Are you thinking
specifically about DM-thinp and its discard_granularity?  DM thinp
doesn't support partial thinp block discards -- so misaligned/partial
discards are dropped by DM thinp.  There isn't a compelling case for
fixing this (by adding a bitset for each thinp block to support finer
grained discards) -- at least not that I'm aware of.

But I'm missing exactly what it is your helper function would do... and
yet you're asking me what my requirements are.. sorry ;)