MFC 303164: Add more documentation regarding unsafe AIO requests.

The asynchronous I/O changes made previously result in different
behavior out of the box. Previously all AIO requests failed with
ENOSYS / SIGSYS unless aio.ko was explicitly loaded. Now, some AIO
requests complete and others ("unsafe" requests) fail with EOPNOTSUPP.

Reword the introductory paragraph in aio(4) to add a general
description of AIO before describing the vfs.aio.enable_unsafe sysctl.

Remove the ENOSYS error description from aio_fsync(2), aio_read(2),
and aio_write(2) and replace it with a description of EOPNOTSUPP.

Remove the ENOSYS error description from aio_mlock(2).

Log a message to the system log the first time a process requests an
"unsafe" AIO request that fails with EOPNOTSUPP. This is modeled on
the log message used for processes using the legacy pty devices.

Approved by:	re (gjb)

4 files changed, 16 insertions, 20 deletions

diff --git a/lib/libc/sys/aio_fsync.2 b/lib/libc/sys/aio_fsync.2
index 7eb1a3b..52b47efb 100644
--- a/lib/libc/sys/aio_fsync.2
+++ b/lib/libc/sys/aio_fsync.2
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 15, 2016
+.Dd July 21, 2016
 .Dt AIO_FSYNC 2
 .Os
 .Sh NAME
@@ -107,10 +107,10 @@ The request was not queued because of system resource limitations.
 The asynchronous notification method in
 .Fa iocb->aio_sigevent.sigev_notify
 is invalid or not supported.
-.It Bq Er ENOSYS
-The
-.Fn aio_fsync
-system call is not supported.
+.It Bq Er EOPNOTSUPP
+Asynchronous file synchronization operations on the file descriptor
+.Fa iocb->aio_fildes
+are unsafe and unsafe asynchronous I/O operations are disabled.
 .It Bq Er EINVAL
 A value of the
 .Fa op
diff --git a/lib/libc/sys/aio_mlock.2 b/lib/libc/sys/aio_mlock.2
index c798a37..cc588b5 100644
--- a/lib/libc/sys/aio_mlock.2
+++ b/lib/libc/sys/aio_mlock.2
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 15, 2016
+.Dd July 21, 2016
 .Dt AIO_MLOCK 2
 .Os
 .Sh NAME
@@ -102,10 +102,6 @@ The request was not queued because of system resource limitations.
 The asynchronous notification method in
 .Fa iocb->aio_sigevent.sigev_notify
 is invalid or not supported.
-.It Bq Er ENOSYS
-The
-.Fn aio_mlock
-system call is not supported.
 .El
 .Pp
 If the request is successfully enqueued, but subsequently cancelled
diff --git a/lib/libc/sys/aio_read.2 b/lib/libc/sys/aio_read.2
index cc0133b..2f16809 100644
--- a/lib/libc/sys/aio_read.2
+++ b/lib/libc/sys/aio_read.2
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 15, 2016
+.Dd July 21, 2016
 .Dt AIO_READ 2
 .Os
 .Sh NAME
@@ -125,10 +125,10 @@ The request was not queued because of system resource limitations.
 The asynchronous notification method in
 .Fa iocb->aio_sigevent.sigev_notify
 is invalid or not supported.
-.It Bq Er ENOSYS
-The
-.Fn aio_read
-system call is not supported.
+.It Bq Er EOPNOTSUPP
+Asynchronous read operations on the file descriptor
+.Fa iocb->aio_fildes
+are unsafe and unsafe asynchronous I/O operations are disabled.
 .El
 .Pp
 The following conditions may be synchronously detected when the
diff --git a/lib/libc/sys/aio_write.2 b/lib/libc/sys/aio_write.2
index 6cbc2ab..414f98f 100644
--- a/lib/libc/sys/aio_write.2
+++ b/lib/libc/sys/aio_write.2
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 15, 2016
+.Dd July 21, 2016
 .Dt AIO_WRITE 2
 .Os
 .Sh NAME
@@ -129,10 +129,10 @@ The request was not queued because of system resource limitations.
 The asynchronous notification method in
 .Fa iocb->aio_sigevent.sigev_notify
 is invalid or not supported.
-.It Bq Er ENOSYS
-The
-.Fn aio_write
-system call is not supported.
+.It Bq Er EOPNOTSUPP
+Asynchronous write operations on the file descriptor
+.Fa iocb->aio_fildes
+are unsafe and unsafe asynchronous I/O operations are disabled.
 .El
 .Pp
 The following conditions may be synchronously detected when the