linux-kernel - Re: [PATCH v5 01/11] block: make generic_make

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Date:	Tue, 11 Aug 2015 10:08:14 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Kent Overstreet <kent.overstreet@...il.com>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Ming Lin <mlin@...nel.org>, axboe@...com,
	Christoph Hellwig <hch@...radead.org>,
	device-mapper development <dm-devel@...hat.com>,
	Ming Lei <ming.lei@...onical.com>,
	Christoph Hellwig <hch@....de>,
	Alasdair Kergon <agk@...hat.com>,
	Lars Ellenberg <drbd-dev@...ts.linbit.com>,
	Philip Kelleher <pjk1939@...ux.vnet.ibm.com>,
	Nitin Gupta <ngupta@...are.org>,
	Ming Lin <ming.l@....samsung.com>,
	Oleg Drokin <oleg.drokin@...el.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jens Axboe <axboe@...nel.dk>,
	Andreas Dilger <andreas.dilger@...el.com>,
	Geoff Levand <geoff@...radead.org>,
	Jiri Kosina <jkosina@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>, Jim Paris <jim@...n.com>,
	Minchan Kim <minchan@...nel.org>,
	Dongsu Park <dpark@...teo.net>, drbd-user@...ts.linbit.com,
	Joe Thornber <ejt@...hat.com>
Subject: Re: [PATCH v5 01/11] block: make generic_make_request handle
 arbitrarily sized bios

On Mon, Aug 10 2015 at 11:38pm -0400,
Kent Overstreet <kent.overstreet@...il.com> wrote:

> On Mon, Aug 10, 2015 at 10:41:55PM -0400, Mike Snitzer wrote:
> > On Mon, Aug 10 2015 at 10:00pm -0400,
> > Martin K. Petersen <martin.petersen@...cle.com> wrote:
> > 
> > > >>>>> "Ming" == Ming Lin <mlin@...nel.org> writes:
> > > 
> > > Ming> Did you mean still use (UINT_MAX >> 9) in blkdev_issue_discard()?
> > > 
> > > Ming> But that doesn't work for dm-thinp. See Kent's suggestion to use
> > > Ming> 1<<31.
> > > 
> > > I'm not sure why things are not working for dm-thinp. Presumably Kent's
> > > code would split the discard at a granularity boundary so why would that
> > > cause problems for dm?
> > 
> > DM-thinp processes discards internally before it passes them down (if
> > configured to do so).  If a discard is smaller than the granularity of a
> > thinp block (whose size is configurable) or if the start and end of the
> > discard's extent is misaligned (relative to the thinp blocks mapped to
> > the logical extent) then the discard won't actually discard partial
> > thinp blocks.
> 
> This kind of logic really doesn't belong in dm - if it's needed, it really
> belongs in bio_split() (which is supposed to work correctly for discards - so if
> it is needed, then bio_split() needs fixing...)

DM thinp does advertise discard_granularity that reflects the thinp blocksize.
blk_queue_split() does look like it'd do the right thing.  But the
splitting that DM thinp is doing is a long standing implementation (in
DM core) that will need to be carefully reviewed/rewritten.  We can
tackle it after all this late splitting code lands.

> IMO though it belongs in the driver - if a discard needs to be dropped because
> it's too small and the hardware can't do it, that should be the driver's
> responsibility.

This isn't about the hardware's limits.  This is about the intermediate
remapping/stacking driver's own space management hooking off of the
discard bio too.
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