mm/damon/core: use mult_frac()

mm/damon/core: use mult_frac()

Patch series "mm/damon: improve/fixup/update ratio calculation, test and
documentation".

Yet another batch of misc/minor improvements and fixups.  Use mult_frac()
instead of the worse open-coding for rate calculations (patch 1).  Add a
test for a previously found and fixed bug (patch 2).  Improve and update
comments and documentations for easier code review and up-to-date
information (patches 3-6).  Finally, fix an obvious typo (patch 7).

This patch (of 7):

There are multiple places in core code that do open-code rate
calculations.  Use mult_frac(), which is developed for doing that in a way
more safe from overflow and precision loss.

Link: https://lkml.kernel.org/r/20260307195356.203753-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20260307195356.203753-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Acked-by: wang lian <lianux.mm@gmail.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>