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+
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