dm snapshot: introduce account_start_copy() and account_end_copy()

BugLink: https://bugs.launchpad.net/bugs/1851549

[ Upstream commit a2f83e8b ]

This simple refactoring moves code for modifying the semaphore cow_count
into separate functions to prepare for changes that will extend these
methods to provide for a more sophisticated mechanism for COW
throttling.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Nikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/md/dm-snap.c

@@ -1400,6 +1400,16 @@ static void snapshot_dtr(struct dm_target *ti)
 	kfree(s);
 }
 
+static void account_start_copy(struct dm_snapshot *s)
+{
+	down(&s->cow_count);
+}
+
+static void account_end_copy(struct dm_snapshot *s)
+{
+	up(&s->cow_count);
+}
+
 /*
  * Flush a list of buffers.
  */
@@ -1584,7 +1594,7 @@ static void copy_callback(int read_err, unsigned long write_err, void *context)
 		}
 		list_add(&pe->out_of_order_entry, lh);
 	}
-	up(&s->cow_count);
+	account_end_copy(s);
 }
 
 /*
@@ -1608,7 +1618,7 @@ static void start_copy(struct dm_snap_pending_exception *pe)
 	dest.count = src.count;
 
 	/* Hand over to kcopyd */
-	down(&s->cow_count);
+	account_start_copy(s);
 	dm_kcopyd_copy(s->kcopyd_client, &src, 1, &dest, 0, copy_callback, pe);
 }
 
@@ -1629,7 +1639,7 @@ static void start_full_bio(struct dm_snap_pending_exception *pe,
 	pe->full_bio_end_io = bio->bi_end_io;
 	pe->full_bio_private = bio->bi_private;
 
-	down(&s->cow_count);
+	account_start_copy(s);
 	callback_data = dm_kcopyd_prepare_callback(s->kcopyd_client,
 						   copy_callback, pe);