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Subject[RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute
From: Ira Weiny <>

In order for users to determine if a file is currently operating in DAX
mode (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


DAX (cpu direct access) is a file mode that attempts to minimize
software cache effects for both I/O and memory mappings of this
file. It requires a capable device, a compatible filesystem
block size, and filesystem opt-in. It generally assumes all
accesses are via cpu load / store instructions which can
minimize overhead for small accesses, but adversely affect cpu
utilization for large transfers. File I/O is done directly
to/from user-space buffers. While the DAX property tends to
result in data being transferred synchronously it does not give
the guarantees of synchronous I/O that data and necessary
metadata are transferred. Memory mapped I/O may be performed
with direct mappings that bypass system memory buffering. Again
while memory-mapped I/O tends to result in data being
transferred synchronously it does not guarantee synchronous
metadata updates. A dax file may optionally support being mapped
with the MAP_SYNC flag which does allow cpu store operations to
be considered synchronous modulo cpu cache effects.

Signed-off-by: Ira Weiny <>
fs/stat.c | 3 +++
include/uapi/linux/stat.h | 1 +
2 files changed, 4 insertions(+)

diff --git a/fs/stat.c b/fs/stat.c
index 030008796479..894699c74dde 100644
--- 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,
diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index ad80a5c885d5..e5f9d5517f6b 100644
--- 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 */
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