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Date: Tue, 27 Mar 2012 11:08:13 +0200
From: Christoph Hellwig <hch@lst.de>
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Subject: Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co,
	aio}_discard for write_zeroes operations
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To: Richard Laager <rlaager@wiktel.com>
Cc: Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org

On Mon, Mar 26, 2012 at 02:40:47PM -0500, Richard Laager wrote:
> On Sat, 2012-03-24 at 16:27 +0100, Christoph Hellwig wrote:
> > >         has_discard = !fallocate(s->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> > 
> > There is no point in using FALLOC_FL_KEEP_SIZE together with
> > FALLOC_FL_PUNCH_HOLE.
> 
> It's *required*. From the man page [0], "The FALLOC_FL_PUNCH_HOLE flag
> must be ORed with FALLOC_FL_KEEP_SIZE in mode;"
> 
> [0] http://man7.org/linux/man-pages/man2/fallocate.2.html

You're right - we check that it is set in common code before entering the
filesystem.

It's utterly pointless, but too late to change now - I should have reviewed
the patch to add it better.