Document special rule when using RWF_ATOMIC flag in conjunction with
O_DIRECT (which is the only time which it can be used).

Some block devices have a virtual boundary, which is a boundary at which
write iovec vectors start and end addresses need to align to - see
virt_boundary_mask in linux kernel sysfs-block Doc.

To avoid splitting writes with torn-write protection in the kernel for
when vectors cross this boundary, special iovec boundary rules are added
such that we don't cross this boundary.

Even though a virt_boundary_mask may be larger than the CPU page size,
we just impose that as a reasonable limit, which matches what the linux
kernel NVMe driver uses today - NVMe is of special interest for supporting
RWF_ATOMIC.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 man2/open.2 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/man2/open.2 b/man2/open.2
index 4c921723c95c..e5b9d107859a 100644
--- a/man2/open.2
+++ b/man2/open.2

@@ -1782,6 +1782,19 @@ blockdev \-\-getss
 .EE
 .in
 .PP
+When
+.B O_DIRECT
+is used in conjunction with
+.BR pwritev2 ()
+and
+.BR RWF_ATOMIC
+flag, in addition to any alignment rules already imposed by the filesystem and
+underlying block device, it must be ensured that
+.I iovec
+vectors have no gaps such that the end alignment of a vector must have the same
+start alignment of any subsequent vector and that alignment must be least
+at a page boundary.
+.PP
 .B O_DIRECT
 I/Os should never be run concurrently with the
 .BR fork (2)

-- 
2.31.1