mm/damon/sysfs-schemes: remove unnecessary schemes existence check in damon_sysfs_schemes_clear_regions()

Patch series "mm/damon: replace most damon_callback usages in sysfs with
new core functions".

DAMON provides damon_callback API that notifies monitoring events and
allows safe access to damon_ctx internal data.  The usage is simple. 
Users register and deregister callback functions for different monitoring
events in damon_ctx.  Then the DAMON worker thread (kdamond) of the
damon_ctx calls back the registered functions on the events.

It is designed in such simple way because it was sufficient for usages of
DAMON at the early days.  We also wanted to make it flexible so that API
user code can implement any required additional features on top of
damon_callback on their demands.

As expected, more sophisticated usages have invented.  Online updates of
DAMON parameters and DAMOS auto-tuning inputs, and online retrieval of
DAMOS statistics and tried regions information are such usages.  Because
damon_callback doesn't provide any explicit synchronization mechanism, the
user ABIs for exposing such functionalities are implemented in
asynchronous ways (DAMON_RECLAIM and DAMON_LRU_SORT}), or synchronous ways
(DAMON_SYSFS) with additional synchronization mechanisms that built inside
the ABI implementation, on top of damon_callback.

So damon_callback is working as expected.  However, the additional
mechanisms built inside ABI on top of damon_callback is becoming somewhat
too big and not easy to maintain.  The additional mechanisms can be
smaller and easier to maintain when implemented inside the core logic
layer.

Introduce two new DAMON core API, namely 'damon_call()' and
'damos_walk()'.  The two functions support synchronous access to
- damon_ctx internal data including DAMON parameters and monitoring
  results, and
- DAMOS-specific data such as regions that each DAMOS action is applied,
respectively.

And replace most of damon_callback usages in DAMON sysfs interface with
the new core API functions.  damon_callback usage for online DAMON
parameters tuning is not replaced in this series, since it has specific
callback timing assumptions that require more works.

Patch sequence
==============

First two patches are fixups for simplifying the following changes.  Those
remove a unnecessary condition check and a synchronization, respectively.

Third patch implements one of the new DAMON core APIs, namely
damon_call().  Three patches replacing damon_callback usages in DAMON
sysfs interface using damon_call() follow.

Then, seventh and eighth patches introduces the other new DAMON API,
damos_walk(), and document it on the design doc.  Ninth patch replaces two
damon_callback usages in DAMON sysfs interface using damos_walk().

The tenth patch finally cleans up code that no more being used.


This patch (of 10):

damon_sysfs_schemes_clear_regions() skips removing the scheme tried region
directories only if the matching scheme is still ongoing.  It is
unnecessary check, since what users want is just removing the entire
region directories.  Remove the unnecessary check.

Link: https://lkml.kernel.org/r/20250103174400.54890-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20250103174400.54890-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

3 files changed, 7 insertions, 14 deletions

diff --git a/mm/damon/sysfs-common.h b/mm/damon/sysfs-common.h
index 9a18f3c535d3..e79b4a65ff2d 100644
--- a/mm/damon/sysfs-common.h
+++ b/mm/damon/sysfs-common.h
@@ -56,8 +56,7 @@ bool damos_sysfs_regions_upd_done(void);
 int damon_sysfs_schemes_update_regions_stop(struct damon_ctx *ctx);
 
 int damon_sysfs_schemes_clear_regions(
-		struct damon_sysfs_schemes *sysfs_schemes,
-		struct damon_ctx *ctx);
+		struct damon_sysfs_schemes *sysfs_schemes);
 
 int damos_sysfs_set_quota_scores(struct damon_sysfs_schemes *sysfs_schemes,
 		struct damon_ctx *ctx);
diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index b095457380b5..2aa34778a472 100644
--- a/mm/damon/sysfs-schemes.c
+++ b/mm/damon/sysfs-schemes.c
@@ -2208,20 +2208,14 @@ void damos_sysfs_mark_finished_regions_updates(struct damon_ctx *ctx)
 
 /* Called from damon_sysfs_cmd_request_callback under damon_sysfs_lock */
 int damon_sysfs_schemes_clear_regions(
-		struct damon_sysfs_schemes *sysfs_schemes,
-		struct damon_ctx *ctx)
+		struct damon_sysfs_schemes *sysfs_schemes)
 {
-	struct damos *scheme;
-	int schemes_idx = 0;
+	int i;
 
-	damon_for_each_scheme(scheme, ctx) {
+	for (i = 0; i < sysfs_schemes->nr; i++) {
 		struct damon_sysfs_scheme *sysfs_scheme;
 
-		/* user could have removed the scheme sysfs dir */
-		if (schemes_idx >= sysfs_schemes->nr)
-			break;
-
-		sysfs_scheme = sysfs_schemes->schemes_arr[schemes_idx++];
+		sysfs_scheme = sysfs_schemes->schemes_arr[i];
 		damon_sysfs_scheme_regions_rm_dirs(
 				sysfs_scheme->tried_regions);
 		sysfs_scheme->tried_regions->total_bytes = 0;
@@ -2271,7 +2265,7 @@ int damon_sysfs_schemes_update_regions_start(
 		struct damon_sysfs_schemes *sysfs_schemes,
 		struct damon_ctx *ctx, bool total_bytes_only)
 {
-	damon_sysfs_schemes_clear_regions(sysfs_schemes, ctx);
+	damon_sysfs_schemes_clear_regions(sysfs_schemes);
 	damon_sysfs_schemes_for_damos_callback = sysfs_schemes;
 	damos_tried_regions_init_upd_status(sysfs_schemes, ctx);
 	damos_regions_upd_total_bytes_only = total_bytes_only;
diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 58145d59881d..789804986ab0 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1262,7 +1262,7 @@ static int damon_sysfs_clear_schemes_regions(
 	if (!ctx)
 		return -EINVAL;
 	return damon_sysfs_schemes_clear_regions(
-			kdamond->contexts->contexts_arr[0]->schemes, ctx);
+			kdamond->contexts->contexts_arr[0]->schemes);
 }
 
 static inline bool damon_sysfs_kdamond_running(