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man/statx: Add STATX_ATTR_DAX

Message ID 20200505002016.1085071-1-ira.weiny@intel.com
State Not Applicable
Headers show
Series man/statx: Add STATX_ATTR_DAX | expand

Commit Message

Ira Weiny May 5, 2020, 12:20 a.m. UTC
From: Ira Weiny <ira.weiny@intel.com>

Linux 5.8 is slated to have STATX_ATTR_DAX support.


Add the text to the statx man page.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
 man2/statx.2 | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
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diff --git a/man2/statx.2 b/man2/statx.2
index 2e90f07dbdbc..14c4ab78e7bd 100644
--- a/man2/statx.2
+++ b/man2/statx.2
@@ -468,6 +468,30 @@  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).
+.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.
+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
+.BR fsync(2).
+.BR mmap(2)
+for more information.
 On success, zero is returned.
 On error, \-1 is returned, and