mm/damon: accept parallel damon_call() requests

mm/damon: accept parallel damon_call() requests

Patch series "mm/damon: remove damon_callback".

damon_callback was the only way for communicating with DAMON for contexts
running on its worker thread.  The interface is flexible and simple.  But
as DAMON evolves with more features, damon_callback has become somewhat
too old.  With runtime parameters update, for example, its lack of
synchronization support was found to be inconvenient.  Arguably it is also
not easy to use correctly since the callers should understand when each
callback is called, and implication of the return values from the
callbacks.

To replace it, damon_call() and damos_walk() are introduced.  And those
replaced a few damon_callback use cases.  Some use cases of damon_callback
such as parallel or repetitive DAMON internal data reading and additional
cleanups cannot simply be replaced by damon_call() and damos_walk(),
though.

To allow those replaceable, extend damon_call() for parallel and/or
repeated callbacks and modify the core/ops layers for additional resources
cleanup.  With the updates, replace the remaining damon_callback usages
and finally say goodbye to damon_callback.

This patch (of 14):

Calling damon_call() while it is serving for another parallel thread
immediately fails with -EBUSY.  The caller should call it again, later.
Each caller implementing such retry logic would be redundant.  Accept
parallel damon_call() requests and do the wait instead of the caller.

Link: https://lkml.kernel.org/r/20250712195016.151108-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20250712195016.151108-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>