Re: [PATCH 0/2] Fix blkdev_issue_discard()

[Jan Kara <jack@suse.cz>]

On Wed 16-12-15 00:12:23, Mike Snitzer wrote:
> On Tue, Dec 15 2015 at  9:38pm -0500,
> Martin K. Petersen <martin.petersen@oracle.com> wrote:
> 
> > >>>>> "Bart" == Bart Van Assche <bart.vanassche@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 ;)

I agree that just dropping head & tail from misaligned discard request
looks reasonable. However I think the original motivation for this thread
was actually blkdev_issue_zeroout(). That ends up being implemented using
blkdev_issue_discard() and in that case silently dropping head and tail is
not an option. Supporting non-aligned blkdev_issue_zeroout() via submitting
properly aligned blkdev_issue_discard() and then just submitting writes
with zeros for head & tail would look useful to me.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR