On Thu, Jan 29, 2015 at 11:02:58AM +0100, Arnd Bergmann wrote:

On Wednesday 28 January 2015 18:00:25 Darrick J. Wong wrote:

quoted

Create a new ioctl to expose the block layer's newfound ability to
issue either a zeroing discard, a WRITE SAME with a zero page, or a
regular write with the zero page.  This BLKZEROOUT2 ioctl takes
{start, length, flags} as parameters.  So far, the only flag available
is to enable the zeroing discard part -- without it, the call invokes
the old BLKZEROOUT behavior.  start and length have the same meaning
as in BLKZEROOUT.

Furthermore, because BLKZEROOUT2 issues commands directly to the
storage device, we must invalidate the page cache (as a regular
O_DIRECT write would do) to avoid returning stale cache contents at a
later time.

This patch depends on "block: Add discard flag to
blkdev_issue_zeroout() function" in Jens' for-3.20/core branch.

Signed-off-by: Darrick J. Wong <redacted>

Would this work ok for devices that fill discarded areas with all-ones
instead of all-zeroes? I believe SD cards can do either.

It won't do all-ones, because the underlying blkdev_issue_zeroout call only
knows how to tell the device to write zeroes or perform a discard if the flag
is set and the device is whitelisted.  This patch only exposes the existing
kernel call to userspace.

(All-ones could be plumbed into the storage stack, but that would have to be a
separate patch.)

--D

	Arnd
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