block, dax: move "select DAX" from BLOCK to FS_DAX

For configurations that do not enable DAX filesystems or drivers, do not
require the DAX core to be built.

Given that the 'direct_access' method has been removed from
'block_device_operations', we can also go ahead and remove the
block-related dax helper functions from fs/block_dev.c to
drivers/dax/super.c. This keeps dax details out of the block layer and
lets the DAX core be built as a module in the FS_DAX=n case.

Filesystems need to include dax.h to call bdev_dax_supported().

Cc: linux-xfs@vger.kernel.org
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

block/Kconfig

@@ -6,7 +6,6 @@ menuconfig BLOCK
        default y
        select SBITMAP
        select SRCU
-       select DAX
        help
 	 Provide block layer support for the kernel.
 

drivers/dax/super.c

@@ -14,6 +14,7 @@
 #include <linux/module.h>
 #include <linux/mount.h>
 #include <linux/magic.h>
+#include <linux/genhd.h>
 #include <linux/cdev.h>
 #include <linux/hash.h>
 #include <linux/slab.h>
@@ -47,6 +48,75 @@ void dax_read_unlock(int id)
 }
 EXPORT_SYMBOL_GPL(dax_read_unlock);
 
+int bdev_dax_pgoff(struct block_device *bdev, sector_t sector, size_t size,
+		pgoff_t *pgoff)
+{
+	phys_addr_t phys_off = (get_start_sect(bdev) + sector) * 512;
+
+	if (pgoff)
+		*pgoff = PHYS_PFN(phys_off);
+	if (phys_off % PAGE_SIZE || size % PAGE_SIZE)
+		return -EINVAL;
+	return 0;
+}
+EXPORT_SYMBOL(bdev_dax_pgoff);
+
+/**
+ * __bdev_dax_supported() - Check if the device supports dax for filesystem
+ * @sb: The superblock of the device
+ * @blocksize: The block size of the device
+ *
+ * This is a library function for filesystems to check if the block device
+ * can be mounted with dax option.
+ *
+ * Return: negative errno if unsupported, 0 if supported.
+ */
+int __bdev_dax_supported(struct super_block *sb, int blocksize)
+{
+	struct block_device *bdev = sb->s_bdev;
+	struct dax_device *dax_dev;
+	pgoff_t pgoff;
+	int err, id;
+	void *kaddr;
+	pfn_t pfn;
+	long len;
+
+	if (blocksize != PAGE_SIZE) {
+		pr_err("VFS (%s): error: unsupported blocksize for dax\n",
+				sb->s_id);
+		return -EINVAL;
+	}
+
+	err = bdev_dax_pgoff(bdev, 0, PAGE_SIZE, &pgoff);
+	if (err) {
+		pr_err("VFS (%s): error: unaligned partition for dax\n",
+				sb->s_id);
+		return err;
+	}
+
+	dax_dev = dax_get_by_host(bdev->bd_disk->disk_name);
+	if (!dax_dev) {
+		pr_err("VFS (%s): error: device does not support dax\n",
+				sb->s_id);
+		return -EOPNOTSUPP;
+	}
+
+	id = dax_read_lock();
+	len = dax_direct_access(dax_dev, pgoff, 1, &kaddr, &pfn);
+	dax_read_unlock(id);
+
+	put_dax(dax_dev);
+
+	if (len < 1) {
+		pr_err("VFS (%s): error: dax access failed (%ld)",
+				sb->s_id, len);
+		return len < 0 ? len : -EIO;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(__bdev_dax_supported);
+
 /**
  * struct dax_device - anchor object for dax services
  * @inode: core vfs

fs/Kconfig

@@ -39,6 +39,7 @@ config FS_DAX
 	depends on MMU
 	depends on !(ARM || MIPS || SPARC)
 	select FS_IOMAP
+	select DAX
 	help
 	  Direct Access (DAX) can be used on memory-backed block devices.
 	  If the block device supports DAX and the filesystem supports DAX,

fs/block_dev.c

@@ -718,72 +718,6 @@ int bdev_write_page(struct block_device *bdev, sector_t sector,
 }
 EXPORT_SYMBOL_GPL(bdev_write_page);
 
-int bdev_dax_pgoff(struct block_device *bdev, sector_t sector, size_t size,
-		pgoff_t *pgoff)
-{
-	phys_addr_t phys_off = (get_start_sect(bdev) + sector) * 512;
-
-	if (pgoff)
-		*pgoff = PHYS_PFN(phys_off);
-	if (phys_off % PAGE_SIZE || size % PAGE_SIZE)
-		return -EINVAL;
-	return 0;
-}
-EXPORT_SYMBOL(bdev_dax_pgoff);
-
-/**
- * bdev_dax_supported() - Check if the device supports dax for filesystem
- * @sb: The superblock of the device
- * @blocksize: The block size of the device
- *
- * This is a library function for filesystems to check if the block device
- * can be mounted with dax option.
- *
- * Return: negative errno if unsupported, 0 if supported.
- */
-int bdev_dax_supported(struct super_block *sb, int blocksize)
-{
-	struct block_device *bdev = sb->s_bdev;
-	struct dax_device *dax_dev;
-	pgoff_t pgoff;
-	int err, id;
-	void *kaddr;
-	pfn_t pfn;
-	long len;
-
-	if (blocksize != PAGE_SIZE) {
-		vfs_msg(sb, KERN_ERR, "error: unsupported blocksize for dax");
-		return -EINVAL;
-	}
-
-	err = bdev_dax_pgoff(bdev, 0, PAGE_SIZE, &pgoff);
-	if (err) {
-		vfs_msg(sb, KERN_ERR, "error: unaligned partition for dax");
-		return err;
-	}
-
-	dax_dev = dax_get_by_host(bdev->bd_disk->disk_name);
-	if (!dax_dev) {
-		vfs_msg(sb, KERN_ERR, "error: device does not support dax");
-		return -EOPNOTSUPP;
-	}
-
-	id = dax_read_lock();
-	len = dax_direct_access(dax_dev, pgoff, 1, &kaddr, &pfn);
-	dax_read_unlock(id);
-
-	put_dax(dax_dev);
-
-	if (len < 1) {
-		vfs_msg(sb, KERN_ERR,
-				"error: dax access failed (%ld)", len);
-		return len < 0 ? len : -EIO;
-	}
-
-	return 0;
-}
-EXPORT_SYMBOL_GPL(bdev_dax_supported);
-
 /*
  * pseudo-fs
  */

fs/ext2/super.c

@@ -32,6 +32,7 @@
 #include <linux/log2.h>
 #include <linux/quotaops.h>
 #include <linux/uaccess.h>
+#include <linux/dax.h>
 #include "ext2.h"
 #include "xattr.h"
 #include "acl.h"

fs/ext4/super.c

@@ -37,6 +37,7 @@
 #include <linux/ctype.h>
 #include <linux/log2.h>
 #include <linux/crc16.h>
+#include <linux/dax.h>
 #include <linux/cleancache.h>
 #include <linux/uaccess.h>
 

fs/xfs/xfs_super.c

@@ -52,6 +52,7 @@
 #include "xfs_reflink.h"
 
 #include <linux/namei.h>
+#include <linux/dax.h>
 #include <linux/init.h>
 #include <linux/slab.h>
 #include <linux/mount.h>

include/linux/blkdev.h

@@ -1940,8 +1940,6 @@ extern int __blkdev_driver_ioctl(struct block_device *, fmode_t, unsigned int,
 extern int bdev_read_page(struct block_device *, sector_t, struct page *);
 extern int bdev_write_page(struct block_device *, sector_t, struct page *,
 						struct writeback_control *);
-extern int bdev_dax_supported(struct super_block *, int);
-int bdev_dax_pgoff(struct block_device *, sector_t, size_t, pgoff_t *pgoff);
 #else /* CONFIG_BLOCK */
 
 struct block_device;

include/linux/dax.h

@@ -18,12 +18,38 @@ struct dax_operations {
 			void **, pfn_t *);
 };
 
+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);
+static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
+{
+	return __bdev_dax_supported(sb, blocksize);
+}
+#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)
+{
+	return NULL;
+}
+
+static inline void put_dax(struct dax_device *dax_dev)
+{
+}
+#endif
+
 int dax_read_lock(void);
 void dax_read_unlock(int id);
-struct dax_device *dax_get_by_host(const char *host);
 struct dax_device *alloc_dax(void *private, const char *host,
 		const struct dax_operations *ops);
-void put_dax(struct dax_device *dax_dev);
 bool dax_alive(struct dax_device *dax_dev);
 void kill_dax(struct dax_device *dax_dev);
 void *dax_get_private(struct dax_device *dax_dev);