statx.2: Add STATX_ATTR_DAX

Linux 5.8 adds STATX_ATTR_DAX support.

https://lore.kernel.org/lkml/20200428002142.404144-4-ira.weiny@intel.com/
https://lore.kernel.org/lkml/20200504161352.GA13783@magnolia/

Add the text to the statx man page.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/statx.2

@@ -462,6 +462,30 @@ The file has fs-verity enabled.
 It cannot be written to, and all reads from it will be verified
 against a cryptographic hash that covers the
 entire file (e.g., via a Merkle tree).
+.TP
+.BR STATX_ATTR_DAX (since Linux 5.8)
+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.
+.PP
+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.
+.PP
+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.
+.PP
+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.
+.PP
+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
+.BR fsync(2).
+See
+.BR mmap(2)
+for more information.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and