[PATCH RFC v2 3/8] dm-table: atomic writes support

[John Garry <john.g.garry@oracle.com>]

Support stacking atomic write limits for DM devices.

All the pre-existing code in blk_stack_atomic_writes_limits() already takes
care of finding the aggregrate limits from the bottom devices.

Feature flag DM_TARGET_ATOMIC_WRITES is introduced so that atomic writes
can be enabled on personalities selectively. This is to ensure that atomic
writes are only enabled when verified to be working properly (for a
specific personality). In addition, it just may not make sense to enable
atomic writes on some personalities (so this flag also helps there).

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 drivers/md/dm-table.c         | 29 +++++++++++++++++++++++++++++
 include/linux/device-mapper.h |  3 +++
 2 files changed, 32 insertions(+)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index bd8b796ae683..0ef5203387b2 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -1806,6 +1806,32 @@ static bool dm_table_supports_secure_erase(struct dm_table *t)
 	return true;
 }
 
+static int device_not_atomic_write_capable(struct dm_target *ti,
+			struct dm_dev *dev, sector_t start,
+			sector_t len, void *data)
+{
+	return !bdev_can_atomic_write(dev->bdev);
+}
+
+static bool dm_table_supports_atomic_writes(struct dm_table *t)
+{
+	for (unsigned int i = 0; i < t->num_targets; i++) {
+		struct dm_target *ti = dm_table_get_target(t, i);
+
+		if (!dm_target_supports_atomic_writes(ti->type))
+			return false;
+
+		if (!ti->type->iterate_devices)
+			return false;
+
+		if (ti->type->iterate_devices(ti,
+			device_not_atomic_write_capable, NULL)) {
+			return false;
+		}
+	}
+	return true;
+}
+
 int dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 			      struct queue_limits *limits)
 {
@@ -1854,6 +1880,9 @@ int dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 			return r;
 	}
 
+	if (dm_table_supports_atomic_writes(t))
+		limits->features |= BLK_FEAT_ATOMIC_WRITES;
+
 	r = queue_limits_set(q, limits);
 	if (r)
 		return r;
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 8321f65897f3..bcc6d7b69470 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -299,6 +299,9 @@ struct target_type {
 #define dm_target_supports_mixed_zoned_model(type) (false)
 #endif
 
+#define DM_TARGET_ATOMIC_WRITES		0x00000400
+#define dm_target_supports_atomic_writes(type) ((type)->features & DM_TARGET_ATOMIC_WRITES)
+
 struct dm_target {
 	struct dm_table *table;
 	struct target_type *type;
-- 
2.31.1