[V7,3/9] fs/stat: Define DAX statx attribute

Commit Message

Ira Weiny April 13, 2020, 5:40 a.m. UTC

From: Ira Weiny <[e-mail]>

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 <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Reviewed-by: Darrick J. Wong <[e-mail]>
Signed-off-by: Ira Weiny <[e-mail]>

---
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(+)

Comments

Christoph Hellwig April 14, 2020, 6:23 a.m. UTC | #1

On Sun, Apr 12, 2020 at 10:40:40PM -0700, [e-mail] wrote:
> 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.

Can we remove the misleading DAX name?  Something like
STATX_ATTR_DIRECT_LOAD_STORE?

Ira Weiny April 14, 2020, 8:39 p.m. UTC | #2

On Tue, Apr 14, 2020 at 08:23:06AM +0200, Christoph Hellwig wrote:
> On Sun, Apr 12, 2020 at 10:40:40PM -0700, [e-mail] wrote:
> > 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.
> 
> Can we remove the misleading DAX name?  Something like
> STATX_ATTR_DIRECT_LOAD_STORE?

This is easy enough to change but...

Honestly I feel like this ship has already sailed.  We have so much out there
which uses the term "DAX".  Is it really better to introduce a new terminology
for the same thing?

Ira

Patch

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 */