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Date: Mon, 10 Aug 2015 22:41:55 -0400
From: Mike Snitzer <snitzer@redhat.com>
To: "Martin K. Petersen" <martin.petersen@oracle.com>
Message-ID: <20150811024154.GA26351@redhat.com>
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Cc: Ming Lei <ming.lei@canonical.com>,
	device-mapper development <dm-devel@redhat.com>,
	Christoph Hellwig <hch@lst.de>, Alasdair Kergon <agk@redhat.com>,
	Lars Ellenberg <drbd-dev@lists.linbit.com>,
	Philip Kelleher <pjk1939@linux.vnet.ibm.com>,
	Christoph Hellwig <hch@infradead.org>,
	Kent Overstreet <kent.overstreet@gmail.com>,
	Nitin Gupta <ngupta@vflare.org>, Ming Lin <ming.l@ssi.samsung.com>,
	Oleg Drokin <oleg.drokin@intel.com>,
	Al Viro <viro@zeniv.linux.org.uk>, Ming Lin <mlin@kernel.org>,
	Jens Axboe <axboe@kernel.dk>, axboe@fb.com,
	Andreas Dilger <andreas.dilger@intel.com>,
	Geoff Levand <geoff@infradead.org>, Jiri Kosina <jkosina@suse.cz>,
	lkml <linux-kernel@vger.kernel.org>, Jim Paris <jim@jtan.com>,
	Minchan Kim <minchan@kernel.org>, Joe Thornber <ejt@redhat.com>,
	Dongsu Park <dpark@posteo.net>, drbd-user@lists.linbit.com
Subject: Re: [Drbd-dev] [PATCH v5 01/11] block: make generic_make_request
 handle arbitrarily sized bios
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On Mon, Aug 10 2015 at 10:00pm -0400,
Martin K. Petersen <martin.petersen@oracle.com> wrote:

> >>>>> "Ming" == Ming Lin <mlin@kernel.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.