mm/damon/reclaim: enable and disable synchronously

Patch series "mm/damon/reclaim,lru_sort: enable/disable synchronously".

Writing a value to DAMON_RECLAIM and DAMON_LRU_SORT's 'enabled' parameters
turns on or off DAMON in an ansychronous way.  This means the parameter
cannot be used to read the current status of them.  'kdamond_pid'
parameter should be used instead for the purpose.  The documentation is
easy to be read as it works in a synchronous way, so it is a little bit
confusing.  It also makes the user space tooling dirty.

There's no real reason to have the asynchronous behavior, though.  Simply
make the parameter works synchronously, rather than updating the document.

The first and second patches changes the behavior of the 'enabled'
parameter for DAMON_RECLAIM and adds a selftest for the changed behavior,
respectively.  Following two patches make the same changes for
DAMON_LRU_SORT.


This patch (of 4):

Writing a value to DAMON_RECLAIM's 'enabled' parameter turns on or off
DAMON in an ansychronous way.  This means the parameter cannot be used to
read the current status of DAMON_RECLAIM.  'kdamond_pid' parameter should
be used instead for the purpose.  The documentation is easy to be read as
it works in a synchronous way, so it is a little bit confusing.  It also
makes the user space tooling dirty.

There's no real reason to have the asynchronous behavior, though.  Simply
make the parameter works synchronously, rather than updating the document.

Link: https://lkml.kernel.org/r/20221025173650.90624-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20221025173650.90624-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>