kernel: avoid palloc lock during mmurelease()

Previously, mmurelease() was always called with
palloc spinlock held.

This is unneccesary for some mmurelease()
implementations as they wont release pages
to the palloc pool.

This change removes pagechainhead() and
pagechaindone() and replaces them with just
freepages() call, which aquires the palloc
lock internally as needed.

freepages() avoids holding the palloc lock
while walking the linked list of pages,
avoding some lock contention.

1 files changed, 9 insertions, 18 deletions

diff --git a/sys/src/9/cycv/mmu.c b/sys/src/9/cycv/mmu.c
index 8a449e70b..213a88178 100644
--- a/sys/src/9/cycv/mmu.c
+++ b/sys/src/9/cycv/mmu.c
@@ -197,23 +197,21 @@ flushmmu(void)
 void
 mmurelease(Proc *proc)
 {
-	Page *p, *n;
+	Page *p;
 
-	if(islo())
-		panic("mmurelease: islo");
-	
 	l1switch(&m->l1, 0);
-	if(proc->kmaptable != nil){
+	if((p = proc->kmaptable) != nil){
+		if(p->ref != 1)
+			panic("mmurelease: kmap ref %ld", p->ref);
 		if(proc->l1 == nil)
 			panic("mmurelease: no l1");
-		if(decref(proc->kmaptable) != 0)
-			panic("mmurelease: kmap ref %ld", proc->kmaptable->ref);
 		if(proc->nkmap)
 			panic("mmurelease: nkmap %d", proc->nkmap);
-		if(PPN(proc->l1->va[L1X(KMAP)]) != proc->kmaptable->pa)
-			panic("mmurelease: bad kmap l2 %#.8lux kmap %#.8lux", proc->l1->va[L1X(KMAP)], proc->kmaptable->pa);
+		if(PPN(proc->l1->va[L1X(KMAP)]) != p->pa)
+			panic("mmurelease: bad kmap l2 %#.8lux kmap %#.8lux", proc->l1->va[L1X(KMAP)], p->pa);
 		proc->l1->va[L1X(KMAP)] = 0;
-		pagechainhead(proc->kmaptable);
+		p->next = proc->mmufree;
+		proc->mmufree = p;
 		proc->kmaptable = nil;
 	}
 	if(proc->l1 != nil){
@@ -221,14 +219,7 @@ mmurelease(Proc *proc)
 		l1free(proc->l1);
 		proc->l1 = nil;
 	}
-	for(p = proc->mmufree; p != nil; p = n){
-		n = p->next;
-		if(decref(p) != 0)
-			panic("mmurelease: p->ref %ld", p->ref);
-		pagechainhead(p);
-	}
-	if(proc->mmufree != nil)
-		pagechaindone();
+	freepages(proc->mmufree, nil, 0);
 	proc->mmufree = nil;
 }