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From: Christoph Hellwig <hch@lst.de>
To: Mike Snitzer <snitzer@redhat.com>
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Cc: axboe@kernel.dk, linux-raid@vger.kernel.org, linux-scsi@vger.kernel.org,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	philipp.reisner@linbit.com, linux-block@vger.kernel.org,
	dm-devel@redhat.com, shli@kernel.org,
	Christoph Hellwig <hch@lst.de>, agk@redhat.com, drbd-dev@lists.linbit.com
Subject: Re: [Drbd-dev] [PATCH 22/23] drbd: implement REQ_OP_WRITE_ZEROES
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On Thu, Mar 30, 2017 at 07:15:50PM -0400, Mike Snitzer wrote:
> I got pretty far along with implementing the DM thinp support for
> WRITE_ZEROES in terms of thinp's DISCARD support (more of an
> implementation detail.. or so I thought).
> 
> But while discussing this effort with Jeff Moyer he asked: shouldn't the
> zeroed blocks be provisioned?  This is a fairly embarassing question not
> to be able to answer in the moment.  So I clearly need to learn what the
> overall intent of WRITE_ZEROES actually is.

It is to ensure the that the block range it's called on returns zeroes
on future reads.  Currently if REQ_UNMAP is set you may free the space
allocation, else not.

After looking at the callers I think this is the wrong way around, so
I'm going to invert the flag, so that the two callers that care that
blocks are allocated will set it instead.