linux-kernel - Re: [PATCH v5.1 0/2] create BLKZEROOUT ioctl that invalidates page cache

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Message-ID: <20160302094416.GA16631@infradead.org>
Date:	Wed, 2 Mar 2016 01:44:16 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"Darrick J. Wong" <darrick.wong@...cle.com>, axboe@...nel.dk,
	hch@...radead.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, martin.petersen@...cle.com,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
	shane.seymour@....com, bfields@...ldses.org,
	linux-fsdevel@...r.kernel.org, jlayton@...chiereds.net
Subject: Re: [PATCH v5.1 0/2] create BLKZEROOUT ioctl that invalidates page
 cache

On Wed, Mar 02, 2016 at 10:15:02AM +0100, Arnd Bergmann wrote:
> I'm missing the background on this, just saw the patch fly by,
> so sorry if this has been asked before:
> 
> Why do you want to invalidate the cache? Is this to save RAM
> or is something else going to write here and you have to invalidate
> it for correctness?

BLKZEROOUT can be though as a direct I/O operation - it issues a 
WRITE SAME (or similar) command straight to the device to zero
the specified range.  If there was cached data for this range it would
now be stale.