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kernel: make pagereclaim() a bit less stupid

put recently used pages at the head of ther image hash
chains, and reclaim pages from the tail first.

--- a/sys/src/9/port/page.c

+++ b/sys/src/9/port/page.c

@@ -86,7 +86,7 @@

 int

 pagereclaim(Image *i, int min)

 {

-	Page **h, **l, *p;

+	Page **h, **l, **x, *p;

 	Page *fh, *ft;

 	int n;

@@ -100,17 +100,18 @@

 	n = 0;

 	fh = ft = nil;

 	for(h = i->pghash; h < &i->pghash[PGHSIZE]; h++){

-		if((p = *h) == nil)

-			continue;

-		for(l = h; p != nil; p = p->next){

+		l = h;

+		x = nil;

+		for(p = *l; p != nil; p = p->next){

 			if(p->ref == 0)

-				break;

+				x = l;

 			l = &p->next;

 		}

-		if(p == nil)

+		if(x == nil)

 			continue;

-		*l = p->next;

+		p = *x;

+		*x = p->next;

 		p->next = nil;

 		p->image = nil;

 		p->daddr = ~0;

@@ -315,15 +316,20 @@

 Page*

 lookpage(Image *i, uintptr daddr)

 {

-	Page *p;

+	Page *p, **h, **l;

 	lock(i);

-	for(p = PGHASH(i, daddr); p != nil; p = p->next) {

-		if(p->daddr == daddr) {

+	l = h = &PGHASH(i, daddr);

+	for(p = *l; p != nil; p = p->next){

+		if(p->daddr == daddr){

+			*l = p->next;

+			p->next = *h;

+			*h = p;

 			incref(p);

 			unlock(i);

 			return p;

 		}

+		l = &p->next;

 	}

 	unlock(i);