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Subject[RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute
Date
From: Ira Weiny <ira.weiny@intel.com>

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
page:

STATX_ATTR_DAX

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 <ira.weiny@intel.com>
---
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,
query_flags);
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 */
--
2.21.0
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