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kernel: duppage cleanup

remove the sched() call and retry loop from duppage() and just
drop the page lock, then reacquire it after lock(&palloc).

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

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

@@ -264,30 +264,12 @@

 	return p;

 }

-static int dupretries = 15000;

-

 void

 duppage(Page *p)				/* Always call with p locked */

 {

 	Page *np;

 	int color;

-	int retries;

-	retries = 0;

-retry:

-	/* don't dup pages that are shared or have no image */

-	if(p->ref != 1 || p->image == nil || p->image->notext)

-		return;

-

-	if(retries++ > dupretries){

-		print("duppage %d, up %p\n", retries, up);

-		dupretries += 100;

-		if(dupretries > 100000)

-			panic("duppage");

-		uncachepage(p);

-		return;

-	}

-

 	/*

 	 *  normal lock ordering is to call

 	 *  lock(&palloc) before lock(p).

@@ -295,14 +277,18 @@

 	 *  our locks and try again. as the page

 	 *  is from the image cache, this might

 	 *  let someone else come in and grab it

-	 *  so we check page ref above.

+	 *  so we check page ref below.

 	 */

 	if(!canlock(&palloc)){

 		unlock(p);

-		if(up)

-			sched();

+		lock(&palloc);

 		lock(p);

-		goto retry;

+	}

+

+	/* don't dup pages that are shared or have no image */

+	if(p->ref != 1 || p->image == nil || p->image->notext){

+		unlock(&palloc);

+		return;

 	}

 	/* No freelist cache when memory is very low */

-- 

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