ext2, ext4, xfs: retrieve dax_device for iomap operations

In preparation for converting fs/dax.c to use dax_direct_access()
instead of bdev_direct_access(), add the plumbing to retrieve the
dax_device associated with a given block_device.
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

fs/ext2/inode.c

@@ -799,6 +799,7 @@ int ext2_get_block(struct inode *inode, sector_t iblock,
 static int ext2_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 		unsigned flags, struct iomap *iomap)
 {
+	struct block_device *bdev;
 	unsigned int blkbits = inode->i_blkbits;
 	unsigned long first_block = offset >> blkbits;
 	unsigned long max_blocks = (length + (1 << blkbits) - 1) >> blkbits;
@@ -812,8 +813,13 @@ static int ext2_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 		return ret;
 
 	iomap->flags = 0;
-	iomap->bdev = inode->i_sb->s_bdev;
+	bdev = inode->i_sb->s_bdev;
+	iomap->bdev = bdev;
 	iomap->offset = (u64)first_block << blkbits;
+	if (blk_queue_dax(bdev->bd_queue))
+		iomap->dax_dev = dax_get_by_host(bdev->bd_disk->disk_name);
+	else
+		iomap->dax_dev = NULL;
 
 	if (ret == 0) {
 		iomap->type = IOMAP_HOLE;
@@ -835,6 +841,7 @@ static int
 ext2_iomap_end(struct inode *inode, loff_t offset, loff_t length,
 		ssize_t written, unsigned flags, struct iomap *iomap)
 {
+	put_dax(iomap->dax_dev);
 	if (iomap->type == IOMAP_MAPPED &&
 	    written < length &&
 	    (flags & IOMAP_WRITE))

fs/ext4/inode.c

@@ -3305,6 +3305,7 @@ static int ext4_releasepage(struct page *page, gfp_t wait)
 static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 			    unsigned flags, struct iomap *iomap)
 {
+	struct block_device *bdev;
 	unsigned int blkbits = inode->i_blkbits;
 	unsigned long first_block = offset >> blkbits;
 	unsigned long last_block = (offset + length - 1) >> blkbits;
@@ -3373,7 +3374,12 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	}
 
 	iomap->flags = 0;
-	iomap->bdev = inode->i_sb->s_bdev;
+	bdev = inode->i_sb->s_bdev;
+	iomap->bdev = bdev;
+	if (blk_queue_dax(bdev->bd_queue))
+		iomap->dax_dev = dax_get_by_host(bdev->bd_disk->disk_name);
+	else
+		iomap->dax_dev = NULL;
 	iomap->offset = first_block << blkbits;
 
 	if (ret == 0) {
@@ -3406,6 +3412,7 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
 	int blkbits = inode->i_blkbits;
 	bool truncate = false;
 
+	put_dax(iomap->dax_dev);
 	if (!(flags & IOMAP_WRITE) || (flags & IOMAP_FAULT))
 		return 0;
 

fs/xfs/xfs_iomap.c

@@ -976,6 +976,7 @@ xfs_file_iomap_begin(
 	int			nimaps = 1, error = 0;
 	bool			shared = false, trimmed = false;
 	unsigned		lockmode;
+	struct block_device	*bdev;
 
 	if (XFS_FORCED_SHUTDOWN(mp))
 		return -EIO;
@@ -1063,6 +1064,14 @@ xfs_file_iomap_begin(
 	}
 
 	xfs_bmbt_to_iomap(ip, iomap, &imap);
+
+	/* optionally associate a dax device with the iomap bdev */
+	bdev = iomap->bdev;
+	if (blk_queue_dax(bdev->bd_queue))
+		iomap->dax_dev = dax_get_by_host(bdev->bd_disk->disk_name);
+	else
+		iomap->dax_dev = NULL;
+
 	if (shared)
 		iomap->flags |= IOMAP_F_SHARED;
 	return 0;
@@ -1140,6 +1149,7 @@ xfs_file_iomap_end(
 	unsigned		flags,
 	struct iomap		*iomap)
 {
+	put_dax(iomap->dax_dev);
 	if ((flags & IOMAP_WRITE) && iomap->type == IOMAP_DELALLOC)
 		return xfs_file_iomap_end_delalloc(XFS_I(inode), offset,
 				length, written, iomap);

include/linux/iomap.h

@@ -41,6 +41,7 @@ struct iomap {
 	u16			type;	/* type of mapping */
 	u16			flags;	/* flags for mapping */
 	struct block_device	*bdev;	/* block device for I/O */
+	struct dax_device	*dax_dev; /* dax_dev for dax operations */
 };
 
 /*