In ibrk and segattach there are two different expressions which check for
expanding or newly created segments overlapping an existing segment.  Neither
version was general enough to catch all possible cases.

This patch also includes a minimal correction for a bug reported
in 9fans by cinap_lenrek on 2014-06-08 (Re: duppage) which can
lead to resource starvation & deadlock:

	duppage() causes the freelist to be shuffled differently. without
	stuffing cached pages at the freelist tail, the tail accumulates
	a uncached "stopper" page which breaks the invariant of imagereclaim
	which just scans from the tail backwards as long as the pages are
	cached.

	imagereclaim does not move the pages to the head after uncaching them!
	so by default imagereclaim prevents the cached pages before the ones
	it reclaimed from being reclaimed ever.

Reference: /n/sources/patch/segment-overlap
Date: Tue Apr 19 13:16:06 CES 2016
Signed-off-by: miller@hamnavoe.com

--- /sys/src/9/port/segment.c	Tue Apr 19 13:06:56 2016
+++ /sys/src/9/port/segment.c	Tue Apr 19 13:06:53 2016
@@ -344,6 +344,8 @@
 			if(p->ref == 0) {
 				n++;
 				uncachepage(p);
+				pageunchain(p);
+				pagechainhead(p);
 			}
 			unlock(p);
 		}
@@ -484,7 +486,7 @@
 		ns = up->seg[i];
 		if(ns == 0 || ns == s)
 			continue;
-		if(newtop >= ns->base && newtop < ns->top) {
+		if(newtop > ns->base && s->base < ns->top) {
 			qunlock(&s->lk);
 			error(Esoverlap);
 		}
@@ -586,8 +588,7 @@
 		ns = p->seg[i];
 		if(ns == 0)
 			continue;
-		if((newtop > ns->base && newtop <= ns->top) ||
-		   (va >= ns->base && va < ns->top))
+		if(newtop > ns->base && va < ns->top)
 			return ns;
 	}
 	return nil;
--- /sys/src/9/port/page.c	Tue Apr 19 13:07:02 2016
+++ /sys/src/9/port/page.c	Tue Apr 19 13:06:59 2016
@@ -64,7 +64,7 @@
 	print("%ldM swap\n", vkb/1024);
 }
 
-static void
+void
 pageunchain(Page *p)
 {
 	if(canlock(&palloc))
--- /sys/src/9/port/portfns.h	Tue Apr 19 13:07:07 2016
+++ /sys/src/9/port/portfns.h	Tue Apr 19 13:07:04 2016
@@ -215,6 +215,7 @@
 void		pageinit(void);
 ulong	pagenumber(Page*);
 void		pagersummary(void);
+void 		pageunchain(Page *p);
 void		panic(char*, ...);
 Cmdbuf*		parsecmd(char *a, int n);
 void		pathclose(Path*);