Commit 0fcb04d5 authored Nov 23, 2015 by Mike Snitzer

dm thin: fix regression in advertised discard limits



When establishing a thin device's discard limits we cannot rely on the
underlying thin-pool device's discard capabilities (which are inherited
from the thin-pool's underlying data device) given that DM thin devices
must provide discard support even when the thin-pool's underlying data
device doesn't support discards.

Users were exposed to this thin device discard limits regression if
their thin-pool's underlying data device does _not_ support discards.
This regression caused all upper-layers that called the
blkdev_issue_discard() interface to not be able to issue discards to
thin devices (because discard_granularity was 0).  This regression
wasn't caught earlier because the device-mapper-test-suite's extensive
'thin-provisioning' discard tests are only ever performed against
thin-pool's with data devices that support discards.

Fix is to have thin_io_hints() test the pool's 'discard_enabled' feature
rather than inferring whether or not a thin device's discard support
should be enabled by looking at the thin-pool's discard_granularity.

Fixes: 21607670 ("dm thin: disable discard support for thin devices if pool's is disabled")
Reported-by: Mike Gerber <mike@sprachgewalt.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 4.1+

parent bcbd94ff

drivers/md/dm-thin.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -4250,10 +4250,9 @@ static void thin_io_hints(struct dm_target ti, struct queue_limits limits)
		{
		struct thin_c *tc = ti->private;
		struct pool *pool = tc->pool;
		struct queue_limits *pool_limits = dm_get_queue_limits(pool->pool_md);

		if (!pool_limits->discard_granularity)
		return; /* pool's discard support is disabled */
		if (!pool->pf.discard_enabled)
		return;

		limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
		limits->max_discard_sectors = 2048 * 1024 * 16; /* 16G */

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