Handle spurious page faults that may occur in no-fault sections of the

kernel.

When access restrictions are added to a page table entry, we flush the
corresponding virtual address mapping from the TLB.  In contrast, when
access restrictions are removed from a page table entry, we do not
flush the virtual address mapping from the TLB.  This is exactly as
recommended in AMD's documentation.  In effect, when access
restrictions are removed from a page table entry, AMD's MMUs will
transparently refresh a stale TLB entry.  In short, this saves us from
having to perform potentially costly TLB flushes.  In contrast,
Intel's MMUs are allowed to generate a spurious page fault based upon
the stale TLB entry.  Usually, such spurious page faults are handled
by vm_fault() without incident.  However, when we are executing
no-fault sections of the kernel, we are not allowed to execute
vm_fault().  This change introduces special-case handling for spurious
page faults that occur in no-fault sections of the kernel.

In collaboration with:	kib
Tested by:		gibbs (an earlier version)

I would also like to acknowledge Hiroki Sato's assistance in
diagnosing this problem.

MFC after:	1 week

1 files changed, 8 insertions, 4 deletions

diff --git a/sys/kern/kern_sysctl.c b/sys/kern/kern_sysctl.c
index 97d03a3..f294ce3 100644
--- a/sys/kern/kern_sysctl.c
+++ b/sys/kern/kern_sysctl.c
@@ -1294,8 +1294,8 @@ kernel_sysctlbyname(struct thread *td, char *name, void *old, size_t *oldlenp,
 static int
 sysctl_old_user(struct sysctl_req *req, const void *p, size_t l)
 {
-	int error = 0;
 	size_t i, len, origidx;
+	int error;
 
 	origidx = req->oldidx;
 	req->oldidx += l;
@@ -1316,10 +1316,14 @@ sysctl_old_user(struct sysctl_req *req, const void *p, size_t l)
 	else {
 		if (i > len - origidx)
 			i = len - origidx;
-		error = copyout(p, (char *)req->oldptr + origidx, i);
+		if (req->lock == REQ_WIRED) {
+			error = copyout_nofault(p, (char *)req->oldptr +
+			    origidx, i);
+		} else
+			error = copyout(p, (char *)req->oldptr + origidx, i);
+		if (error != 0)
+			return (error);
 	}
-	if (error)
-		return (error);
 	if (i < l)
 		return (ENOMEM);
 	return (0);