mm/damon: remove ->before_start of damon_callback

The function pointer field was added to be used as a place to do some
initialization works just before DAMON starts working.  However, nobody is
using it now.  Remove it.

Link: https://lkml.kernel.org/r/20250306175908.66300-11-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

@@ -603,16 +603,14 @@ struct damon_operations {
 /**
  * struct damon_callback - Monitoring events notification callbacks.
  *
- * @before_start:	Called before starting the monitoring.
  * @after_wmarks_check:	Called after each schemes' watermarks check.
  * @after_sampling:	Called after each sampling.
  * @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_start and
- * @before_terminate just before starting and finishing the monitoring,
- * respectively.
+ * The monitoring thread (&damon_ctx.kdamond) calls @before_terminate just
+ * before finishing the monitoring.
  *
  * The monitoring thread calls @after_wmarks_check after each DAMON-based
  * operation schemes' watermarks check.  If users need to make changes to the
@@ -628,7 +626,6 @@ struct damon_operations {
  * If any callback returns non-zero, monitoring stops.
  */
 struct damon_callback {
-	int (*before_start)(struct damon_ctx *context);
 	int (*after_wmarks_check)(struct damon_ctx *context);
 	int (*after_sampling)(struct damon_ctx *context);
 	int (*after_aggregation)(struct damon_ctx *context);

mm/damon/core.c

@@ -2408,8 +2408,6 @@ static int kdamond_fn(void *data)
 
 	if (ctx->ops.init)
 		ctx->ops.init(ctx);
-	if (ctx->callback.before_start && ctx->callback.before_start(ctx))
-		goto done;
 	ctx->regions_score_histogram = kmalloc_array(DAMOS_MAX_SCORE + 1,
 			sizeof(*ctx->regions_score_histogram), GFP_KERNEL);
 	if (!ctx->regions_score_histogram)