From d3e4d71f1dbe57d2779a96ec2d4f4f09f8a8bb5c Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Fri, 28 Oct 2016 11:53:22 +0000
Subject: [PATCH] Handle pmap_enter() over an existing 4/2M page in KVA on
 i386.

The userspace case was already handled by pmap_allocpte().  For kernel
VA, page table page must exist, and demote cannot fail, so we need to
just call pmap_demote_pde().  Also note that due to the machine AS
layout, promotions in the KVA on i386 are highly unlikely, so this
change is mostly for completeness.

Reviewed by:	alc, markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D8323
---
 sys/i386/i386/pmap.c | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/sys/i386/i386/pmap.c b/sys/i386/i386/pmap.c
index 7c2d56abac9..765ff8bec51 100644
--- a/sys/i386/i386/pmap.c
+++ b/sys/i386/i386/pmap.c
@@ -3466,11 +3466,14 @@ pmap_enter(pmap_t pmap, vm_offset_t va, vm_page_t m, vm_prot_t prot,
 	PMAP_LOCK(pmap);
 	sched_pin();
 
-	/*
-	 * In the case that a page table page is not
-	 * resident, we are creating it here.
-	 */
+	pde = pmap_pde(pmap, va);
 	if (va < VM_MAXUSER_ADDRESS) {
+		/*
+		 * va is for UVA.
+		 * In the case that a page table page is not resident,
+		 * we are creating it here.  pmap_allocpte() handles
+		 * demotion.
+		 */
 		mpte = pmap_allocpte(pmap, va, flags);
 		if (mpte == NULL) {
 			KASSERT((flags & PMAP_ENTER_NOSLEEP) != 0,
@@ -3480,19 +3483,28 @@ pmap_enter(pmap_t pmap, vm_offset_t va, vm_page_t m, vm_prot_t prot,
 			PMAP_UNLOCK(pmap);
 			return (KERN_RESOURCE_SHORTAGE);
 		}
+	} else {
+		/*
+		 * va is for KVA, so pmap_demote_pde() will never fail
+		 * to install a page table page.  PG_V is also
+		 * asserted by pmap_demote_pde().
+		 */
+		KASSERT(pde != NULL && (*pde & PG_V) != 0,
+		    ("KVA %#x invalid pde pdir %#jx", va,
+		    (uintmax_t)pmap->pm_pdir[PTDPTDI]));
+		if ((*pde & PG_PS) != 0)
+			pmap_demote_pde(pmap, pde, va);
 	}
-
-	pde = pmap_pde(pmap, va);
-	if ((*pde & PG_PS) != 0)
-		panic("pmap_enter: attempted pmap_enter on 4MB page");
 	pte = pmap_pte_quick(pmap, va);
 
 	/*
-	 * Page Directory table entry not valid, we need a new PT page
+	 * Page Directory table entry is not valid, which should not
+	 * happen.  We should have either allocated the page table
+	 * page or demoted the existing mapping above.
 	 */
 	if (pte == NULL) {
 		panic("pmap_enter: invalid page directory pdir=%#jx, va=%#x",
-			(uintmax_t)pmap->pm_pdir[PTDPTDI], va);
+		    (uintmax_t)pmap->pm_pdir[PTDPTDI], va);
 	}
 
 	pa = VM_PAGE_TO_PHYS(m);