dm: add dax_device and dax_operations support

Allocate a dax_device to represent the capacity of a device-mapper
instance. Provide a ->direct_access() method via the new dax_operations
indirection that mirrors the functionality of the current direct_access
support via block_device_operations.  Once fs/dax.c has been converted
to use dax_operations the old dm_blk_direct_access() will be removed.

A new helper dm_dax_get_live_target() is introduced to separate some of
the dm-specifics from the direct_access implementation.

This enabling is only for the top-level dm representation to upper
layers. Converting target direct_access implementations is deferred to a
separate patch.

Cc: Toshi Kani <toshi.kani@hpe.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/md/Kconfig

@@ -200,6 +200,7 @@ config BLK_DEV_DM_BUILTIN
 config BLK_DEV_DM
 	tristate "Device mapper support"
 	select BLK_DEV_DM_BUILTIN
+	select DAX
 	---help---
 	  Device-mapper is a low level volume manager.  It works by allowing
 	  people to specify mappings for ranges of logical sectors.  Various

drivers/md/dm-core.h

@@ -58,6 +58,7 @@ struct mapped_device {
 	struct target_type *immutable_target_type;
 
 	struct gendisk *disk;
+	struct dax_device *dax_dev;
 	char name[16];
 
 	void *interface_ptr;

drivers/md/dm.c

@@ -16,6 +16,7 @@
 #include <linux/blkpg.h>
 #include <linux/bio.h>
 #include <linux/mempool.h>
+#include <linux/dax.h>
 #include <linux/slab.h>
 #include <linux/idr.h>
 #include <linux/hdreg.h>
@@ -908,31 +909,68 @@ int dm_set_target_max_io_len(struct dm_target *ti, sector_t len)
 }
 EXPORT_SYMBOL_GPL(dm_set_target_max_io_len);
 
+static struct dm_target *dm_dax_get_live_target(struct mapped_device *md,
+		sector_t sector, int *srcu_idx)
+{
+	struct dm_table *map;
+	struct dm_target *ti;
+
+	map = dm_get_live_table(md, srcu_idx);
+	if (!map)
+		return NULL;
+
+	ti = dm_table_find_target(map, sector);
+	if (!dm_target_is_valid(ti))
+		return NULL;
+
+	return ti;
+}
+
+static long dm_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
+		long nr_pages, void **kaddr, pfn_t *pfn)
+{
+	struct mapped_device *md = dax_get_private(dax_dev);
+	sector_t sector = pgoff * PAGE_SECTORS;
+	struct dm_target *ti;
+	long len, ret = -EIO;
+	int srcu_idx;
+
+	ti = dm_dax_get_live_target(md, sector, &srcu_idx);
+
+	if (!ti)
+		goto out;
+	if (!ti->type->direct_access)
+		goto out;
+	len = max_io_len(sector, ti) / PAGE_SECTORS;
+	if (len < 1)
+		goto out;
+	nr_pages = min(len, nr_pages);
+	if (ti->type->direct_access) {
+		ret = ti->type->direct_access(ti, sector, kaddr, pfn,
+				nr_pages * PAGE_SIZE);
+		/*
+		 * FIXME: convert ti->type->direct_access to return
+		 * nr_pages directly.
+		 */
+		if (ret >= 0)
+			ret /= PAGE_SIZE;
+	}
+ out:
+	dm_put_live_table(md, srcu_idx);
+
+	return ret;
+}
+
 static long dm_blk_direct_access(struct block_device *bdev, sector_t sector,
 		void **kaddr, pfn_t *pfn, long size)
 {
 	struct mapped_device *md = bdev->bd_disk->private_data;
-	struct dm_table *map;
-	struct dm_target *ti;
-	int srcu_idx;
-	long len, ret = -EIO;
+	struct dax_device *dax_dev = md->dax_dev;
+	long nr_pages = size / PAGE_SIZE;
 
-	map = dm_get_live_table(md, &srcu_idx);
-	if (!map)
-		goto out;
-
-	ti = dm_table_find_target(map, sector);
-	if (!dm_target_is_valid(ti))
-		goto out;
-
-	len = max_io_len(sector, ti) << SECTOR_SHIFT;
-	size = min(len, size);
-
-	if (ti->type->direct_access)
-		ret = ti->type->direct_access(ti, sector, kaddr, pfn, size);
-out:
-	dm_put_live_table(md, srcu_idx);
-	return min(ret, size);
+	nr_pages = dm_dax_direct_access(dax_dev, sector / PAGE_SECTORS,
+			nr_pages, kaddr, pfn);
+	return nr_pages < 0 ? nr_pages : nr_pages * PAGE_SIZE;
 }
 
 /*
@@ -1437,6 +1475,7 @@ static int next_free_minor(int *minor)
 }
 
 static const struct block_device_operations dm_blk_dops;
+static const struct dax_operations dm_dax_ops;
 
 static void dm_wq_work(struct work_struct *work);
 
@@ -1483,6 +1522,12 @@ static void cleanup_mapped_device(struct mapped_device *md)
 	if (md->bs)
 		bioset_free(md->bs);
 
+	if (md->dax_dev) {
+		kill_dax(md->dax_dev);
+		put_dax(md->dax_dev);
+		md->dax_dev = NULL;
+	}
+
 	if (md->disk) {
 		spin_lock(&_minor_lock);
 		md->disk->private_data = NULL;
@@ -1510,6 +1555,7 @@ static void cleanup_mapped_device(struct mapped_device *md)
 static struct mapped_device *alloc_dev(int minor)
 {
 	int r, numa_node_id = dm_get_numa_node();
+	struct dax_device *dax_dev;
 	struct mapped_device *md;
 	void *old_md;
 
@@ -1574,6 +1620,12 @@ static struct mapped_device *alloc_dev(int minor)
 	md->disk->queue = md->queue;
 	md->disk->private_data = md;
 	sprintf(md->disk->disk_name, "dm-%d", minor);
+
+	dax_dev = alloc_dax(md, md->disk->disk_name, &dm_dax_ops);
+	if (!dax_dev)
+		goto bad;
+	md->dax_dev = dax_dev;
+
 	add_disk(md->disk);
 	format_dev_t(md->name, MKDEV(_major, minor));
 
@@ -2781,6 +2833,10 @@ static const struct block_device_operations dm_blk_dops = {
 	.owner = THIS_MODULE
 };
 
+static const struct dax_operations dm_dax_ops = {
+	.direct_access = dm_dax_direct_access,
+};
+
 /*
  * module hooks
  */

include/linux/device-mapper.h

@@ -130,6 +130,7 @@ typedef int (*dm_busy_fn) (struct dm_target *ti);
  */
 typedef long (*dm_direct_access_fn) (struct dm_target *ti, sector_t sector,
 				     void **kaddr, pfn_t *pfn, long size);
+#define PAGE_SECTORS (PAGE_SIZE / 512)
 
 void dm_error(const char *message);