From 0d2e1c3e39359263dc3342b3105db2bec383e3cd Mon Sep 17 00:00:00 2001
From: Alan Cox <alc@FreeBSD.org>
Date: Sun, 25 Apr 2010 20:40:45 +0000
Subject: [PATCH] Clearing a page table entry's accessed bit (PG_A) and setting
 the page's PG_REFERENCED flag in pmap_protect() can't really be justified. In
 contrast to pmap_remove() or pmap_remove_all(), the mapping is not being
 destroyed, so the notion that the page was accessed is not lost. Moreover,
 clearing the page table entry's accessed bit and setting the page's
 PG_REFERENCED flag can throw off the page daemon's activity count
 calculation.  Finally, in my tests, I found that 15% of the atomic memory
 operations being performed by pmap_protect() were only to clear PG_A, and not
 change protection.  This could, by itself, be fixed, but I don't see the
 point given the above argument.

Remove a comment from pmap_protect_pde() that is no longer meaningful
after the above change.
---
 sys/amd64/amd64/pmap.c | 29 ++++++-----------------------
 sys/i386/i386/pmap.c   | 28 ++++++----------------------
 2 files changed, 12 insertions(+), 45 deletions(-)

diff --git a/sys/amd64/amd64/pmap.c b/sys/amd64/amd64/pmap.c
index 9c7d8b088a9c..6cd10b476a84 100644
--- a/sys/amd64/amd64/pmap.c
+++ b/sys/amd64/amd64/pmap.c
@@ -2833,18 +2833,9 @@ pmap_protect_pde(pmap_t pmap, pd_entry_t *pde, vm_offset_t sva, vm_prot_t prot)
 	if (oldpde & PG_MANAGED) {
 		eva = sva + NBPDR;
 		for (va = sva, m = PHYS_TO_VM_PAGE(oldpde & PG_PS_FRAME);
-		    va < eva; va += PAGE_SIZE, m++) {
-			/*
-			 * In contrast to the analogous operation on a 4KB page
-			 * mapping, the mapping's PG_A flag is not cleared and
-			 * the page's PG_REFERENCED flag is not set.  The
-			 * reason is that pmap_demote_pde() expects that a 2MB
-			 * page mapping with a stored page table page has PG_A
-			 * set.
-			 */
+		    va < eva; va += PAGE_SIZE, m++)
 			if ((oldpde & (PG_M | PG_RW)) == (PG_M | PG_RW))
 				vm_page_dirty(m);
-		}
 	}
 	if ((prot & VM_PROT_WRITE) == 0)
 		newpde &= ~(PG_RW | PG_M);
@@ -2953,23 +2944,15 @@ pmap_protect(pmap_t pmap, vm_offset_t sva, vm_offset_t eva, vm_prot_t prot)
 			obits = pbits = *pte;
 			if ((pbits & PG_V) == 0)
 				continue;
-			if (pbits & PG_MANAGED) {
-				m = NULL;
-				if (pbits & PG_A) {
+
+			if ((prot & VM_PROT_WRITE) == 0) {
+				if ((pbits & (PG_MANAGED | PG_M | PG_RW)) ==
+				    (PG_MANAGED | PG_M | PG_RW)) {
 					m = PHYS_TO_VM_PAGE(pbits & PG_FRAME);
-					vm_page_flag_set(m, PG_REFERENCED);
-					pbits &= ~PG_A;
-				}
-				if ((pbits & (PG_M | PG_RW)) == (PG_M | PG_RW)) {
-					if (m == NULL)
-						m = PHYS_TO_VM_PAGE(pbits &
-						    PG_FRAME);
 					vm_page_dirty(m);
 				}
-			}
-
-			if ((prot & VM_PROT_WRITE) == 0)
 				pbits &= ~(PG_RW | PG_M);
+			}
 			if ((prot & VM_PROT_EXECUTE) == 0)
 				pbits |= pg_nx;
 
diff --git a/sys/i386/i386/pmap.c b/sys/i386/i386/pmap.c
index 1ce6454ffbb1..d8b9686e7383 100644
--- a/sys/i386/i386/pmap.c
+++ b/sys/i386/i386/pmap.c
@@ -2955,18 +2955,9 @@ pmap_protect_pde(pmap_t pmap, pd_entry_t *pde, vm_offset_t sva, vm_prot_t prot)
 	if (oldpde & PG_MANAGED) {
 		eva = sva + NBPDR;
 		for (va = sva, m = PHYS_TO_VM_PAGE(oldpde & PG_PS_FRAME);
-		    va < eva; va += PAGE_SIZE, m++) {
-			/*
-			 * In contrast to the analogous operation on a 4KB page
-			 * mapping, the mapping's PG_A flag is not cleared and
-			 * the page's PG_REFERENCED flag is not set.  The
-			 * reason is that pmap_demote_pde() expects that a 2/4MB
-			 * page mapping with a stored page table page has PG_A
-			 * set.
-			 */
+		    va < eva; va += PAGE_SIZE, m++)
 			if ((oldpde & (PG_M | PG_RW)) == (PG_M | PG_RW))
 				vm_page_dirty(m);
-		}
 	}
 	if ((prot & VM_PROT_WRITE) == 0)
 		newpde &= ~(PG_RW | PG_M);
@@ -3074,22 +3065,15 @@ pmap_protect(pmap_t pmap, vm_offset_t sva, vm_offset_t eva, vm_prot_t prot)
 			obits = pbits = *pte;
 			if ((pbits & PG_V) == 0)
 				continue;
-			if (pbits & PG_MANAGED) {
-				m = NULL;
-				if (pbits & PG_A) {
+
+			if ((prot & VM_PROT_WRITE) == 0) {
+				if ((pbits & (PG_MANAGED | PG_M | PG_RW)) ==
+				    (PG_MANAGED | PG_M | PG_RW)) {
 					m = PHYS_TO_VM_PAGE(pbits & PG_FRAME);
-					vm_page_flag_set(m, PG_REFERENCED);
-					pbits &= ~PG_A;
-				}
-				if ((pbits & (PG_M | PG_RW)) == (PG_M | PG_RW)) {
-					if (m == NULL)
-						m = PHYS_TO_VM_PAGE(pbits & PG_FRAME);
 					vm_page_dirty(m);
 				}
-			}
-
-			if ((prot & VM_PROT_WRITE) == 0)
 				pbits &= ~(PG_RW | PG_M);
+			}
 #ifdef PAE
 			if ((prot & VM_PROT_EXECUTE) == 0)
 				pbits |= pg_nx;