[PATCH v6] dax: introduce IS_DEVDAX() and IS_FSDAX()

[Dan Williams]

The current IS_DAX() helper that checks if a file is in DAX mode serves
two purposes. It is a control flow branch condition for DAX vs
non-DAX paths and it is a mechanism to perform dead code elimination. The
dead code elimination is required in the CONFIG_FS_DAX=n case since
there are symbols in fs/dax.c that will be elided. While the
dead code elimination can be addressed with nop stubs for the fs/dax.c
symbols that does not address the need for a DAX control flow helper
where fs/dax.c symbols are not involved.

Moreover, the control flow changes, in some cases, need to be cognizant
of whether the DAX file is a typical file or a Device-DAX special file.
Introduce IS_DEVDAX() and IS_FSDAX() to simultaneously address the
file-type control flow and dead-code elimination use cases. IS_DAX()
will be deleted after all sites are converted to use the file-type
specific helper.

Note, this change is also a pre-requisite for fixing the definition of
the S_DAX inode flag in the CONFIG_FS_DAX=n + CONFIG_DEV_DAX=y case.
The flag needs to be defined, non-zero, if either DAX facility is
enabled.

Cc: "Theodore Ts'o" <tytso@xxxxxxx>
Cc: Andreas Dilger <adilger.kernel@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
Cc: linux-xfs@xxxxxxxxxxxxxxx
Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: dee410792419 ("/dev/dax, core: file operations and dax-mmap")
Reported-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
Changes since v5:
* add comments to clarify the S_ISCHR() checks (Darrick)

 include/linux/fs.h |   24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 79c413985305..751975b8b29b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1909,6 +1909,30 @@ static inline bool sb_rdonly(const struct super_block *sb) { return sb->s_flags
 #define IS_WHITEOUT(inode)	(S_ISCHR(inode->i_mode) && \
 				 (inode)->i_rdev == WHITEOUT_DEV)
 
+static inline bool IS_DEVDAX(struct inode *inode)
+{
+	if (!IS_ENABLED(CONFIG_DEV_DAX))
+		return false;
+	if ((inode->i_flags & S_DAX) == 0)
+		return false;
+	/* regular files with S_DAX are filesystem-dax instances */
+	if (!S_ISCHR(inode->i_mode))
+		return false;
+	return true;
+}
+
+static inline bool IS_FSDAX(struct inode *inode)
+{
+	if (!IS_ENABLED(CONFIG_FS_DAX))
+		return false;
+	if ((inode->i_flags & S_DAX) == 0)
+		return false;
+	/* character devices with S_DAX are device-dax instances */
+	if (S_ISCHR(inode->i_mode))
+		return false;
+	return true;
+}
+
 static inline bool HAS_UNMAPPED_ID(struct inode *inode)
 {
 	return !uid_valid(inode->i_uid) || !gid_valid(inode->i_gid);