From: Ira Weiny <ira.weiny@xxxxxxxxx> Linux 5.8 is slated to have STATX_ATTR_DAX support. https://lore.kernel.org/lkml/20200428002142.404144-4-ira.weiny@xxxxxxxxx/ https://lore.kernel.org/lkml/20200504161352.GA13783@magnolia/ Add the text to the statx man page. Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx> --- man2/statx.2 | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) 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). +.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 -- 2.25.1
