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Subject[PATCH V6 3/8] 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
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: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

---
Changes from V2:
Update man page text with comments from Darrick, Jan, Dan, and
Dave.
---
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 */
--
2.25.1
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