fs/stat: Define DAX statx attribute

In order for users to determine if a file is currently operating in DAX
state (effective DAX).  Define a statx attribute value and set that
attribute if the effective DAX flag is set.

To go along with this we propose the following addition to the statx man
page:

STATX_ATTR_DAX

	The file is in the DAX (cpu direct access) state.  DAX state
	attempts to minimize software cache effects for both I/O and
	memory mappings of this file.  It requires a file system which
	has been configured to support DAX.

	DAX generally assumes all accesses are via cpu load / store
	instructions which can minimize overhead for small accesses, but
	may adversely affect cpu utilization for large transfers.

	File I/O is done directly to/from user-space buffers and memory
	mapped I/O may be performed with direct memory mappings that
	bypass kernel page cache.

	While the DAX property tends to result in data being transferred
	synchronously, it does not give the same guarantees of O_SYNC
	where data and the necessary metadata are transferred together.

	A DAX file may support being mapped with the MAP_SYNC flag,
	which enables a program to use CPU cache flush instructions to
	persist CPU store operations without an explicit fsync(2).  See
	mmap(2) for more information.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

diff --git a/fs/stat.c b/fs/stat.c
index 030008796479aa135964995e4fe8a1fad91ede37..894699c74dded1f8820c6c9bc9cb99bf9dc72228 100644 (file)
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -79,6 +79,9 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
        if (IS_AUTOMOUNT(inode))
                stat->attributes |= STATX_ATTR_AUTOMOUNT;
 
+       if (IS_DAX(inode))
+               stat->attributes |= STATX_ATTR_DAX;
+
        if (inode->i_op->getattr)
                return inode->i_op->getattr(path, stat, request_mask,
                                            query_flags);

diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index ad80a5c885d598231ccf207ac48d2d908eaf1b85..e5f9d5517f6ba524e3593a9d6ef459960d9f4c5f 100644 (file)
--- a/include/uapi/linux/stat.h
+++ b/include/uapi/linux/stat.h
@@ -169,6 +169,7 @@ struct statx {
 #define STATX_ATTR_ENCRYPTED           0x00000800 /* [I] File requires key to decrypt in fs */
 #define STATX_ATTR_AUTOMOUNT           0x00001000 /* Dir: Automount trigger */
 #define STATX_ATTR_VERITY              0x00100000 /* [I] Verity protected file */
+#define STATX_ATTR_DAX                 0x00002000 /* [I] File is DAX */
 
 
 #endif /* _UAPI_LINUX_STAT_H */