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segment: speed up fixedseg() doing single pass over freelist

walking the freelist for every page is too slow. as we
are freeing a range, we can do a single pass unlinking all
pages in our range and at the end, check if all pages
where freed, if not put the pages that we did free back
and retry, otherwise we'r done.

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

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

@@ -580,8 +580,8 @@

 {

 	KMap *k;

 	Segment *s;

-	Page **f, *p;

-	ulong n, i, j;

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

+	ulong n, i;

 	int color;

 	s = newseg(SG_FIXED, va, len);

@@ -591,10 +591,10 @@

 	}

 	lock(&palloc);

 	i = 0;

-	p = palloc.pages;

+	l = palloc.pages;

 	color = getpgcolor(va);

-	for(n = palloc.user; n >= len; n--, p++){

-		if(p->ref != 0 || i != 0 && (p[-1].pa+BY2PG) != p->pa || i == 0 && p->color != color){

+	for(n = palloc.user; n >= len; n--, l++){

+		if(l->ref != 0 || i != 0 && (l[-1].pa+BY2PG) != l->pa || i == 0 && l->color != color){

 		Retry:

 			i = 0;

 			continue;

@@ -601,22 +601,34 @@

 		}

 		if(++i < len)

 			continue;

-		for(j = 0; j < i; j++, p--){

-			for(f = &palloc.head; *f != nil; f = &((*f)->next)){

-				if(*f == p){

-					*f = p->next;

-					goto Freed;

-				}

+

+		i = 0;

+		h = nil;

+		f = &palloc.head;

+		while((p = *f) != nil){

+			if(p > &l[-len] && p <= l){

+				*f = p->next;

+				p->next = h;

+				h = p;

+				if(++i < len)

+					continue;

+				break;

 			}

-			while(j-- > 0)

-				pagechainhead(++p);

+			f = &p->next;

+		}

+		palloc.freecount -= i;

+

+		if(i != len){

+			while((p = h) != nil){

+				h = h->next;

+				pagechainhead(p);

+			}

 			goto Retry;

-		Freed:

-			palloc.freecount--;

 		}

 		unlock(&palloc);

-		while(i-- > 0){

+		p = &l[-len];

+		do {

 			p++;

 			p->ref = 1;

 			p->va = va;

@@ -629,7 +641,7 @@

 			segpage(s, p);

 			va += BY2PG;

-		}

+		} while(p != l);

 		poperror();

 		return s;

 	}