mm/damon/dbgfs: allow users to set initial monitoring target regions

Patch series "DAMON: Support Physical Memory Address Space Monitoring:.

DAMON currently supports only virtual address spaces monitoring.  It can
be easily extended for various use cases and address spaces by
configuring its monitoring primitives layer to use appropriate
primitives implementations, though.  This patchset implements monitoring
primitives for the physical address space monitoring using the
structure.

The first 3 patches allow the user space users manually set the
monitoring regions.  The 1st patch implements the feature in the
'damon-dbgfs'.  Then, patches for adding a unit tests (the 2nd patch)
and updating the documentation (the 3rd patch) follow.

Following 4 patches implement the physical address space monitoring
primitives.  The 4th patch makes some primitive functions for the
virtual address spaces primitives reusable.  The 5th patch implements
the physical address space monitoring primitives.  The 6th patch links
the primitives to the 'damon-dbgfs'.  Finally, 7th patch documents this
new features.

This patch (of 7):

Some 'damon-dbgfs' users would want to monitor only a part of the entire
virtual memory address space.  The program interface users in the kernel
space could use '->before_start()' callback or set the regions inside
the context struct as they want, but 'damon-dbgfs' users cannot.

For that reason, this introduces a new debugfs file called
'init_region'.  'damon-dbgfs' users can specify which initial monitoring
target address regions they want by writing special input to the file.
The input should describe each region in each line in the below form:

    <pid> <start address> <end address>

Note that the regions will be updated to cover entire memory mapped
regions after a 'regions update interval' is passed.  If you want the
regions to not be updated after the initial setting, you could set the
interval as a very long time, say, a few decades.

Link: https://lkml.kernel.org/r/20211012205711.29216-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20211012205711.29216-2-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Amit Shah <amit@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Woodhouse <dwmw@amazon.com>
Cc: Marco Elver <elver@google.com>
Cc: Leonard Foerster <foersleo@amazon.de>
Cc: Greg Thelen <gthelen@google.com>
Cc: Markus Boehme <markubo@amazon.de>
Cc: David Rienjes <rientjes@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>