mm/damon: introduce damon_set_region_system_rams_default()

Patch series "mm/damon/reclaim,lru_sort: monitor all system rams by
default".

DAMON_RECLAIM and DAMON_LRU_SORT set the biggest 'System RAM' resource of
the system as the default monitoring target address range.  The main
intention behind the design is to minimize the overhead coming from
monitoring of non-System RAM areas.

This could result in an odd setup when there are multiple discrete System
RAMs of considerable sizes.  For example, there are System RAMs each
having 500 GiB size.  In this case, only the first 500 GiB will be set as
the monitoring region by default.  This is particularly common on NUMA
systems.  Hence the modules allow users to set the monitoring target
address range using the module parameters if the default setup doesn't
work for them.  In other words, the current design trades ease of setup
for lower overhead.

However, because DAMON utilizes the sampling based access check and the
adaptive regions adjustment mechanisms, the overhead from the monitoring
of non-System RAM areas should be negligible in most setups.  Meanwhile,
the setup complexity is causing real headaches for users who need to run
those modules on various types of systems.  That is, the current tradeoff
is not a good deal.

Set the physical address range that can cover all System RAM areas of the
system as the default monitoring regions for DAMON_RECLAIM and
DAMON_LRU_SORT.

Technically speaking, this is changing documented behavior.  However, it
makes no sense to believe there is a real use case that really depends on
the old weird default behavior.  If the old default behavior was working
for them in the reasonable way, this change will only add a negligible
amount of monitoring overhead.  If it didn't work, the users may already
be using manual monitoring regions setup, and they will not be affected by
this change.

Patches Sequence
================

Patch 1 introduces a new core function that will be used for the new
default monitoring target region setup.  Patch 2 and 3 update
DAMON_RECLAIM and DAMON_LRU_SORT to use the new function instead of the
old one, respectively.  Patch 4 removes the old core function that was
replaced by the new one, as there is no more user of it.  Patch 5 updates
DAMON_STAT to use the new one instead of its in-house nearly-duplicate
self implementation of the functionality.  Finally patches 6 and 7 update
the DAMON_RECLAIM and DAMON_LRU_SORT user documentation for the new
behaviors, respectively.


This patch (of 7):

damon_set_region_biggest_system_ram_default() sets the monitoring target
region as the caller requested.  If the caller didn't specify the region,
it finds the biggest System RAM of the system and sets it as the target
region.  When there are more than one considerable size of System RAM
resources in the system, the default target setup makes no sense. 
Introduce a variant, namely damon_set_region_system_rams_default().  It
sets a physical address range that covers all System RAM resources as the
default target region.

Link: https://lore.kernel.org/20260429041232.90257-1-sj@kernel.org
Link: https://lore.kernel.org/20260429041232.90257-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/damon.h b/include/linux/damon.h
index f2370a3a4a9a..f656908b2d38 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -1010,6 +1010,11 @@ int damon_kdamond_pid(struct damon_ctx *ctx);
 int damon_call(struct damon_ctx *ctx, struct damon_call_control *control);
 int damos_walk(struct damon_ctx *ctx, struct damos_walk_control *control);
 
+int damon_set_region_system_rams_default(struct damon_target *t,
+				unsigned long *start, unsigned long *end,
+				unsigned long addr_unit,
+				unsigned long min_region_sz);
+
 int damon_set_region_biggest_system_ram_default(struct damon_target *t,
 				unsigned long *start, unsigned long *end,
 				unsigned long addr_unit,