[PATCH v4 2/3] readv.2: Document RWF_ATOMIC flag

John Garry
2024-07-17T09:36:51Z

From: Himanshu Madhani <himanshu.madhani@oracle.com>

Add RWF_ATOMIC flag description for pwritev2().

Signed-off-by: Himanshu Madhani <himanshu.madhani@oracle.com>
[jpg: complete rewrite]
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
man/man2/readv.2 | 76 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 76 insertions(+)

diff --git a/man/man2/readv.2 b/man/man2/readv.2
index eecde06dc..9c8a11324 100644
--- a/man/man2/readv.2
+++ b/man/man2/readv.2
@@ -193,6 +193,66 @@ which provides lower latency, but may use additional resources.
.B O_DIRECT
flag.)
.TP
+.BR RWF_ATOMIC " (since Linux 6.11)"
+Requires that writes to regular files in block-based filesystems be issued with
+torn-write protection.
+Torn-write protection means that for a power or any other hardware failure,
+all or none of the data from the write will be stored,
+but never a mix of old and new data.
+This flag is meaningful only for
+.BR pwritev2 (),
+and its effect applies only to the data range written by the system call.
+The total write length must be power-of-2 and must be sized in the range
+.RI [ stx_atomic_write_unit_min ,
+.IR stx_atomic_write_unit_max ].
+The write must be at a naturally-aligned offset within the file with respect to
+the total write length -
+for example,
+a write of length 32KB at a file offset of 32KB is permitted,
+however a write of length 32KB at a file offset of 48KB is not permitted.
+The upper limit of
+.I iovcnt
+for
+.BR pwritev2 ()
+is in
+.I stx_atomic_write_segments_max.
+Torn-write protection only works with
+.B O_DIRECT
+flag, i.e. buffered writes are not supported.
+To guarantee consistency from the write between a file's in-core state with the
+storage device,
+.BR fdatasync (2),
+or
+.BR fsync (2),
+or
+.BR open (2)
+and either
+.B O_SYNC
+or
+.B O_DSYNC,
+or
+.B pwritev2 ()
+and either
+.B RWF_SYNC
+or
+.B RWF_DSYNC
+is required. Flags
+.B O_SYNC
+or
+.B RWF_SYNC
+provide the strongest guarantees for
+.BR RWF_ATOMIC,
+in that all data and also file metadata updates will be persisted for a
+successfully completed write.
+Just using either flags
+.B O_DSYNC
+or
+.B RWF_DSYNC
+means that all data and any file updates will be persisted for a successfully
+completed write.
+Not using any sync flags means that there is no guarantee that data or
+filesystem updates are persisted.
+.TP
.BR RWF_SYNC " (since Linux 4.7)"
.\" commit e864f39569f4092c2b2bc72c773b6e486c7e3bd9
Provide a per-write equivalent of the
@@ -279,10 +339,26 @@ values overflows an
.I ssize_t
value.
.TP
+.B EINVAL
+ For
+.BR RWF_ATOMIC
+set,
+the combination of the sum of the
+.I iov_len
+values and the
+.I offset
+value does not comply with the length and offset torn-write protection rules.
+.TP
.B EINVAL
The vector count,
.IR iovcnt ,
is less than zero or greater than the permitted maximum.
+For
+.BR RWF_ATOMIC
+set, this maximum is in
+.I stx_atomic_write_segments_max
+from
+.I statx.
.TP
.B EOPNOTSUPP
An unknown flag is specified in \fIflags\fP.
--
2.31.1