dax, xfs, ext4: compile out iomap-dax paths in the FS_DAX=n case

Tetsuo reports:

  fs/built-in.o: In function `xfs_file_iomap_end':
  xfs_iomap.c:(.text+0xe0ef9): undefined reference to `put_dax'
  fs/built-in.o: In function `xfs_file_iomap_begin':
  xfs_iomap.c:(.text+0xe1a7f): undefined reference to `dax_get_by_host'
  make: *** [vmlinux] Error 1
  $ grep DAX .config
  CONFIG_DAX=m
  # CONFIG_DEV_DAX is not set
  # CONFIG_FS_DAX is not set

When FS_DAX=n we can/must throw away the dax code in filesystems.
Implement 'fs_' versions of dax_get_by_host() and put_dax() that are
nops in the FS_DAX=n case.

Cc: <linux-xfs@vger.kernel.org>
Cc: <linux-ext4@vger.kernel.org>
Cc: Jan Kara <jack@suse.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Tested-by: Tony Luck <tony.luck@intel.com>
Fixes: ef51042472f5 ("block, dax: move 'select DAX' from BLOCK to FS_DAX")
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

fs/ext2/inode.c

@@ -817,7 +817,7 @@ static int ext2_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	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);
+		iomap->dax_dev = fs_dax_get_by_host(bdev->bd_disk->disk_name);
 	else
 		iomap->dax_dev = NULL;
 
@@ -841,7 +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);
+	fs_put_dax(iomap->dax_dev);
 	if (iomap->type == IOMAP_MAPPED &&
 	    written < length &&
 	    (flags & IOMAP_WRITE))

fs/ext4/inode.c

@@ -3412,7 +3412,7 @@ retry:
 	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);
+		iomap->dax_dev = fs_dax_get_by_host(bdev->bd_disk->disk_name);
 	else
 		iomap->dax_dev = NULL;
 	iomap->offset = first_block << blkbits;
@@ -3447,7 +3447,7 @@ 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);
+	fs_put_dax(iomap->dax_dev);
 	if (!(flags & IOMAP_WRITE) || (flags & IOMAP_FAULT))
 		return 0;
 

fs/xfs/xfs_iomap.c

@@ -1068,7 +1068,7 @@ xfs_file_iomap_begin(
 	/* 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);
+		iomap->dax_dev = fs_dax_get_by_host(bdev->bd_disk->disk_name);
 	else
 		iomap->dax_dev = NULL;
 
@@ -1149,7 +1149,7 @@ xfs_file_iomap_end(
 	unsigned		flags,
 	struct iomap		*iomap)
 {
-	put_dax(iomap->dax_dev);
+	fs_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/dax.h

@@ -18,6 +18,20 @@ struct dax_operations {
 			void **, pfn_t *);
 };
 
+#if IS_ENABLED(CONFIG_DAX)
+struct dax_device *dax_get_by_host(const char *host);
+void put_dax(struct dax_device *dax_dev);
+#else
+static inline struct dax_device *dax_get_by_host(const char *host)
+{
+	return NULL;
+}
+
+static inline void put_dax(struct dax_device *dax_dev)
+{
+}
+#endif
+
 int bdev_dax_pgoff(struct block_device *, sector_t, size_t, pgoff_t *pgoff);
 #if IS_ENABLED(CONFIG_FS_DAX)
 int __bdev_dax_supported(struct super_block *sb, int blocksize);
@@ -25,23 +39,29 @@ static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
 {
 	return __bdev_dax_supported(sb, blocksize);
 }
+
+static inline struct dax_device *fs_dax_get_by_host(const char *host)
+{
+	return dax_get_by_host(host);
+}
+
+static inline void fs_put_dax(struct dax_device *dax_dev)
+{
+	put_dax(dax_dev);
+}
+
 #else
 static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
 {
 	return -EOPNOTSUPP;
 }
-#endif
 
-#if IS_ENABLED(CONFIG_DAX)
-struct dax_device *dax_get_by_host(const char *host);
-void put_dax(struct dax_device *dax_dev);
-#else
-static inline struct dax_device *dax_get_by_host(const char *host)
+static inline struct dax_device *fs_dax_get_by_host(const char *host)
 {
 	return NULL;
 }
 
-static inline void put_dax(struct dax_device *dax_dev)
+static inline void fs_put_dax(struct dax_device *dax_dev)
 {
 }
 #endif