[RFC PATCH 2/4] mm/damon/core: Allow non-exclusive DAMON start/stop

[SeongJae Park <sj@kernel.org>]

To avoid interference between DAMON users, current start/stop functions
of DAMON allows only exclusive groups.  This makes use of DAMON a little
bit restrictive.  Meanwhile, admins could somehow aware each DAMON usage
and therefore could address the interference on their own.  This commit
hence allows callers of the functions to specify if the contexts should
run in an exclusive mode or not.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h |  2 +-
 mm/damon/core.c       | 22 ++++++++++++++++------
 mm/damon/dbgfs.c      |  2 +-
 mm/damon/reclaim.c    |  2 +-
 4 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 49c4a11ecf20..f8e99e47d747 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -508,7 +508,7 @@ int damon_nr_running_ctxs(void);
 int damon_register_ops(struct damon_operations *ops);
 int damon_select_ops(struct damon_ctx *ctx, enum damon_ops_id id);
 
-int damon_start(struct damon_ctx **ctxs, int nr_ctxs);
+int damon_start(struct damon_ctx **ctxs, int nr_ctxs, bool exclusive);
 int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
 
 #endif	/* CONFIG_DAMON */
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 82e0a4620c4f..08c5e4dd2ed9 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -24,6 +24,7 @@
 
 static DEFINE_MUTEX(damon_lock);
 static int nr_running_ctxs;
+static bool running_exclusive_ctxs;
 
 static DEFINE_MUTEX(damon_ops_lock);
 static struct damon_operations damon_registered_ops[NR_DAMON_OPS];
@@ -434,22 +435,25 @@ static int __damon_start(struct damon_ctx *ctx)
  * damon_start() - Starts the monitorings for a given group of contexts.
  * @ctxs:	an array of the pointers for contexts to start monitoring
  * @nr_ctxs:	size of @ctxs
+ * @exclusive:	exclusiveness of this contexts group
  *
  * This function starts a group of monitoring threads for a group of monitoring
  * contexts.  One thread per each context is created and run in parallel.  The
- * caller should handle synchronization between the threads by itself.  If a
- * group of threads that created by other 'damon_start()' call is currently
- * running, this function does nothing but returns -EBUSY.
+ * caller should handle synchronization between the threads by itself.  If
+ * @exclusive is true and a group of threads that created by other
+ * 'damon_start()' call is currently running, this function does nothing but
+ * returns -EBUSY.
  *
  * Return: 0 on success, negative error code otherwise.
  */
-int damon_start(struct damon_ctx **ctxs, int nr_ctxs)
+int damon_start(struct damon_ctx **ctxs, int nr_ctxs, bool exclusive)
 {
 	int i;
 	int err = 0;
 
 	mutex_lock(&damon_lock);
-	if (nr_running_ctxs) {
+	if ((exclusive && nr_running_ctxs) ||
+			(!exclusive && running_exclusive_ctxs)) {
 		mutex_unlock(&damon_lock);
 		return -EBUSY;
 	}
@@ -460,13 +464,15 @@ int damon_start(struct damon_ctx **ctxs, int nr_ctxs)
 			break;
 		nr_running_ctxs++;
 	}
+	if (exclusive && nr_running_ctxs)
+		running_exclusive_ctxs = true;
 	mutex_unlock(&damon_lock);
 
 	return err;
 }
 
 /*
- * __damon_stop() - Stops monitoring of given context.
+ * __damon_stop() - Stops monitoring of a given context.
  * @ctx:	monitoring context
  *
  * Return: 0 on success, negative error code otherwise.
@@ -506,6 +512,10 @@ int damon_stop(struct damon_ctx **ctxs, int nr_ctxs)
 		if (err)
 			return err;
 	}
+	mutex_lock(&damon_lock);
+	if (!nr_running_ctxs && running_exclusive_ctxs)
+		running_exclusive_ctxs = false;
+	mutex_unlock(&damon_lock);
 
 	return err;
 }
diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index 05b574cbcea8..a0dab8b5e45f 100644
--- a/mm/damon/dbgfs.c
+++ b/mm/damon/dbgfs.c
@@ -967,7 +967,7 @@ static ssize_t dbgfs_monitor_on_write(struct file *file,
 				return -EINVAL;
 			}
 		}
-		ret = damon_start(dbgfs_ctxs, dbgfs_nr_ctxs);
+		ret = damon_start(dbgfs_ctxs, dbgfs_nr_ctxs, true);
 	} else if (!strncmp(kbuf, "off", count)) {
 		ret = damon_stop(dbgfs_ctxs, dbgfs_nr_ctxs);
 	} else {
diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index b53d9c22fad1..e34c4d0c4d93 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -330,7 +330,7 @@ static int damon_reclaim_turn(bool on)
 	if (err)
 		goto free_scheme_out;
 
-	err = damon_start(&ctx, 1);
+	err = damon_start(&ctx, 1, true);
 	if (!err) {
 		kdamond_pid = ctx->kdamond->pid;
 		return 0;
-- 
2.17.1