mm/damon/core: safely handle no region case in damon_set_regions()

mm/damon/core: safely handle no region case in damon_set_regions()

Patch series "mm/damon: minor improvements for code readability and tests".

Implement minor improvements on code readability and tests for DAMON.

First seven patches are for DAMON code readability and resulting
maintenance.  Patches 1 and 2 make damon_set_regions() safer and easier to
read.  Patches 3 and 4 remove fragmented DAMON API use cases.  Patches 5-7
hides unused core functions that are unnecessarily exposed to API callers.

The following seven patches are for DAMON tests improvement.  Patches 8
and 9 adds and removes DAMON_DEBUG_SANITY verifications to ensure
reasonable test coverage without too high overhead.  Patch 10 adds a new
kunit test for damon_set_regions().  Patch 11 makes sysfs.py selftest more
gracefully finishes under test failures.  Patches 12-13 adds simple
sysfs.sh test cases for the monitoring intervals goal directory, the
addr_unit file and the pause file.

This patch (of 14):

damon_set_regions() calls damon_first_region() regardless of the number of
DAMON regions in a given DAMON target.  damon_first_region() internally
uses list_first_entry(), which clearly documents the list is expected to
be not empty.  Due to the internal implementation of the macro,
damon_set_regions() is safe for now.  But the internal implementation of
the macro can be changed in future.  Refactor the function to explicitly
and safely handle the empty region list case without depending on the
internal implementation.

No behavioral change is intended.

Link: https://lore.kernel.org/20260522154026.80546-1-sj@kernel.org
Link: https://lore.kernel.org/20260522154026.80546-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>