dm, dax: Make sure dm_dax_flush() is called if device supports it

Currently dm_dax_flush() is not being called, even if underlying dax
device supports write cache, because DAXDEV_WRITE_CACHE is not being
propagated up to the DM dax device.

If the underlying dax device supports write cache, set
DAXDEV_WRITE_CACHE on the DM dax device. This will cause dm_dax_flush()
to be called.

Fixes: abebfbe2f7 ("dm: add ->flush() dax operation support")
Signed-off-by: Vivek Goyal <[e-mail]>
Acked-by: Dan Williams <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/dax/super.c

···
 }
 EXPORT_SYMBOL_GPL(dax_write_cache);
 
+bool dax_write_cache_enabled(struct dax_device *dax_dev)
+{
+	return test_bit(DAXDEV_WRITE_CACHE, &dax_dev->flags);
+}
+EXPORT_SYMBOL_GPL(dax_write_cache_enabled);
+
 bool dax_alive(struct dax_device *dax_dev)
 {
 	lockdep_assert_held(&dax_srcu);

drivers/md/dm-table.c

···
 #include <linux/atomic.h>
 #include <linux/blk-mq.h>
 #include <linux/mount.h>
+#include <linux/dax.h>
 
 #define DM_MSG_PREFIX "table"
 
···
 	return false;
 }
 
+static int device_dax_write_cache_enabled(struct dm_target *ti,
+					  struct dm_dev *dev, sector_t start,
+					  sector_t len, void *data)
+{
+	struct dax_device *dax_dev = dev->dax_dev;
+
+	if (!dax_dev)
+		return false;
+
+	if (dax_write_cache_enabled(dax_dev))
+		return true;
+	return false;
+}
+
+static int dm_table_supports_dax_write_cache(struct dm_table *t)
+{
+	struct dm_target *ti;
+	unsigned i;
+
+	for (i = 0; i < dm_table_get_num_targets(t); i++) {
+		ti = dm_table_get_target(t, i);
+
+		if (ti->type->iterate_devices &&
+		    ti->type->iterate_devices(ti,
+				device_dax_write_cache_enabled, NULL))
+			return true;
+	}
+
+	return false;
+}
+
 static int device_is_nonrot(struct dm_target *ti, struct dm_dev *dev,
 			    sector_t start, sector_t len, void *data)
 {
···
 			fua = true;
 	}
 	blk_queue_write_cache(q, wc, fua);
+
+	if (dm_table_supports_dax_write_cache(t))
+		dax_write_cache(t->md->dax_dev, true);
 
 	/* Ensure that all underlying devices are non-rotational. */
 	if (dm_table_all_devices_attribute(t, device_is_nonrot))

include/linux/dax.h

···
 void dax_flush(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
 		size_t size);
 void dax_write_cache(struct dax_device *dax_dev, bool wc);
+bool dax_write_cache_enabled(struct dax_device *dax_dev);
 
 /*
  * We use lowest available bit in exceptional entry for locking, one bit for