mm/damon: remove the target id concept

DAMON asks each monitoring target ('struct damon_target') to have one
'unsigned long' integer called 'id', which should be unique among the
targets of same monitoring context.  Meaning of it is, however, totally up
to the monitoring primitives that registered to the monitoring context.
For example, the virtual address spaces monitoring primitives treats the
id as a 'struct pid' pointer.

This makes the code flexible, but ugly, not well-documented, and
type-unsafe[1].  Also, identification of each target can be done via its
index.  For the reason, this commit removes the concept and uses clear
type definition.  For now, only 'struct pid' pointer is used for the
virtual address spaces monitoring.  If DAMON is extended in future so that
we need to put another identifier field in the struct, we will use a union
for such primitives-dependent fields and document which primitives are
using which type.

[1] https://lore.kernel.org/linux-mm/20211013154535.4aaeaaf9d0182922e405dd1e@linux-foundation.org/

Link: https://lkml.kernel.org/r/20211230100723.2238-5-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 6 deletions

diff --git a/include/linux/damon.h b/include/linux/damon.h
index bd021af5db3d..7c1d915b3587 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -60,19 +60,18 @@ struct damon_region {
 
 /**
  * struct damon_target - Represents a monitoring target.
- * @id:			Unique identifier for this target.
+ * @pid:		The PID of the virtual address space to monitor.
  * @nr_regions:		Number of monitoring target regions of this target.
  * @regions_list:	Head of the monitoring target regions of this target.
  * @list:		List head for siblings.
  *
  * Each monitoring context could have multiple targets.  For example, a context
  * for virtual memory address spaces could have multiple target processes.  The
- * @id of each target should be unique among the targets of the context.  For
- * example, in the virtual address monitoring context, it could be a pidfd or
- * an address of an mm_struct.
+ * @pid should be set for appropriate address space monitoring primitives
+ * including the virtual address spaces monitoring primitives.
  */
 struct damon_target {
-	unsigned long id;
+	struct pid *pid;
 	unsigned int nr_regions;
 	struct list_head regions_list;
 	struct list_head list;
@@ -475,7 +474,7 @@ struct damos *damon_new_scheme(
 void damon_add_scheme(struct damon_ctx *ctx, struct damos *s);
 void damon_destroy_scheme(struct damos *s);
 
-struct damon_target *damon_new_target(unsigned long id);
+struct damon_target *damon_new_target(void);
 void damon_add_target(struct damon_ctx *ctx, struct damon_target *t);
 bool damon_targets_empty(struct damon_ctx *ctx);
 void damon_free_target(struct damon_target *t);