mm/damon: remove damon_callback->before_damos_apply

The hook was introduced to let DAMON kernel API users access DAMOS
schemes-eligible regions in a safe way.  Now it is no more used by anyone,
and the functionality is provided in a better way by damos_walk().  Remove
it.

Link: https://lkml.kernel.org/r/20250306175908.66300-13-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sjp38

include/linux/damon.h

@@ -605,7 +605,6 @@ struct damon_operations {
  *
  * @after_wmarks_check:	Called after each schemes' watermarks check.
  * @after_aggregation:	Called after each aggregation.
- * @before_damos_apply:	Called before applying DAMOS action.
  * @before_terminate:	Called before terminating the monitoring.
  *
  * The monitoring thread (&damon_ctx.kdamond) calls @before_terminate just
@@ -626,10 +625,6 @@ struct damon_operations {
 struct damon_callback {
 	int (*after_wmarks_check)(struct damon_ctx *context);
 	int (*after_aggregation)(struct damon_ctx *context);
-	int (*before_damos_apply)(struct damon_ctx *context,
-			struct damon_target *target,
-			struct damon_region *region,
-			struct damos *scheme);
 	void (*before_terminate)(struct damon_ctx *context);
 };
 

mm/damon/core.c

@@ -1731,7 +1731,6 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 	struct timespec64 begin, end;
 	unsigned long sz_applied = 0;
 	unsigned long sz_ops_filter_passed = 0;
-	int err = 0;
 	/*
 	 * We plan to support multiple context per kdamond, as DAMON sysfs
 	 * implies with 'nr_contexts' file.  Nevertheless, only single context
@@ -1771,14 +1770,10 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 		if (damos_filter_out(c, t, r, s))
 			return;
 		ktime_get_coarse_ts64(&begin);
-		if (c->callback.before_damos_apply)
-			err = c->callback.before_damos_apply(c, t, r, s);
-		if (!err) {
-			trace_damos_before_apply(cidx, sidx, tidx, r,
-					damon_nr_regions(t), do_trace);
-			sz_applied = c->ops.apply_scheme(c, t, r, s,
-					&sz_ops_filter_passed);
-		}
+		trace_damos_before_apply(cidx, sidx, tidx, r,
+				damon_nr_regions(t), do_trace);
+		sz_applied = c->ops.apply_scheme(c, t, r, s,
+				&sz_ops_filter_passed);
 		damos_walk_call_walk(c, t, r, s, sz_ops_filter_passed);
 		ktime_get_coarse_ts64(&end);
 		quota->total_charged_ns += timespec64_to_ns(&end) -