[PATCH v3 2/6] dax: fix dax_mapping() definition in the FS_DAX=n + DEV_DAX=y case

[Dan Williams <dan.j.williams@intel.com>]

An address_space will only have dax exceptional entries when FS_DAX is
enabled. The current reliance on S_DAX causes compile failures when
S_DAX is defined for DEV_DAX, but FS_DAX is disabled. Make dax_mapping()
always return false so that mm/truncate.c drops its link time
dependencies on fs/dax.c.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>
Cc: <stable@vger.kernel.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: dee410792419 ("/dev/dax, core: file operations and dax-mmap")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 include/linux/dax.h |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/dax.h b/include/linux/dax.h
index 0185ecdae135..62e8cf7eb566 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -107,6 +107,10 @@ int dax_invalidate_mapping_entry_sync(struct address_space *mapping,
 int __dax_zero_page_range(struct block_device *bdev,
 		struct dax_device *dax_dev, sector_t sector,
 		unsigned int offset, unsigned int length);
+static inline bool dax_mapping(struct address_space *mapping)
+{
+	return mapping->host && IS_DAX(mapping->host);
+}
 #else
 static inline int __dax_zero_page_range(struct block_device *bdev,
 		struct dax_device *dax_dev, sector_t sector,
@@ -114,12 +118,11 @@ static inline int __dax_zero_page_range(struct block_device *bdev,
 {
 	return -ENXIO;
 }
-#endif
-
 static inline bool dax_mapping(struct address_space *mapping)
 {
-	return mapping->host && IS_DAX(mapping->host);
+	return false;
 }
+#endif
 
 struct writeback_control;
 int dax_writeback_mapping_range(struct address_space *mapping,