mm, memory_hotplug: be more verbose for memory offline failures

mm, memory_hotplug: be more verbose for memory offline failures

There is only very limited information printed when the memory offlining
fails:

[ 1984.506184] rac1 kernel: memory offlining [mem 0x82600000000-0x8267fffffff] failed due to signal backoff

This tells us that the failure is triggered by the userspace intervention
but it doesn't tell us much more about the underlying reason.  It might be
that the page migration failes repeatedly and the userspace timeout
expires and send a signal or it might be some of the earlier steps
(isolation, memory notifier) takes too long.

If the migration failes then it would be really helpful to see which page
that and its state.  The same applies to the isolation phase.  If we fail
to isolate a page from the allocator then knowing the state of the page
would be helpful as well.

Dump the page state that fails to get isolated or migrated.  This will
tell us more about the failure and what to focus on during debugging.

[[email protected]: add missing printk arg]
[[email protected]: tweak dump_page() `reason' text]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: Anshuman Khandual <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: William Kucharski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>