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)

1 files changed, 10 insertions, 1 deletions

diff --git a/sys/kern/vfs_aio.c b/sys/kern/vfs_aio.c
index 1c75ff9..ead5e7c 100644
--- a/sys/kern/vfs_aio.c
+++ b/sys/kern/vfs_aio.c
@@ -53,6 +53,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/syscall.h>
 #include <sys/sysent.h>
 #include <sys/sysctl.h>
+#include <sys/syslog.h>
 #include <sys/sx.h>
 #include <sys/taskqueue.h>
 #include <sys/vnode.h>
@@ -110,6 +111,11 @@ static int enable_aio_unsafe = 0;
 SYSCTL_INT(_vfs_aio, OID_AUTO, enable_unsafe, CTLFLAG_RW, &enable_aio_unsafe, 0,
     "Permit asynchronous IO on all file types, not just known-safe types");
 
+static unsigned int unsafe_warningcnt = 1;
+SYSCTL_UINT(_vfs_aio, OID_AUTO, unsafe_warningcnt, CTLFLAG_RW,
+    &unsafe_warningcnt, 0,
+    "Warnings that will be triggered upon failed IO requests on unsafe files");
+
 static int max_aio_procs = MAX_AIO_PROCS;
 SYSCTL_INT(_vfs_aio, OID_AUTO, max_aio_procs, CTLFLAG_RW, &max_aio_procs, 0,
     "Maximum number of kernel processes to use for handling async IO ");
@@ -1697,8 +1703,11 @@ queueit:
 				safe = true;
 		}
 	}
-	if (!(safe || enable_aio_unsafe))
+	if (!(safe || enable_aio_unsafe)) {
+		counted_warning(&unsafe_warningcnt,
+		    "is attempting to use unsafe AIO requests");
 		return (EOPNOTSUPP);
+	}
 
 	if (opcode == LIO_SYNC) {
 		AIO_LOCK(ki);