[PATCH] prevent NULL mmap in topdown model

[Rik van Riel <riel@redhat.com>]

This (trivial) patch prevents the topdown allocator from allocating
mmap areas all the way down to address zero.  It's not the prettiest
patch, so suggestions for improvement are welcome ;)

Signed-off-by: Rik van Riel <riel@redhat.com>

--- linux-2.6.11/mm/mmap.c.nullptr	2005-05-17 23:07:26.000000000 -0400
+++ linux-2.6.11/mm/mmap.c	2005-05-17 23:18:53.000000000 -0400
@@ -1251,7 +1251,7 @@
 	addr = mm->free_area_cache;
 
 	/* make sure it can fit in the remaining address space */
-	if (addr >= len) {
+	if (addr >= (len + mm->brk)) {
 		vma = find_vma(mm, addr-len);
 		if (!vma || addr <= vma->vm_start)
 			/* remember the address as a hint for next time */
@@ -1267,13 +1267,13 @@
 		 * return with success:
 		 */
 		vma = find_vma(mm, addr);
-		if (!vma || addr+len <= vma->vm_start)
+		if ((!vma || addr+len <= vma->vm_start) && addr > mm->brk)
 			/* remember the address as a hint for next time */
 			return (mm->free_area_cache = addr);
 
 		/* try just below the current vma->vm_start */
 		addr = vma->vm_start-len;
-	} while (len <= vma->vm_start);
+	} while (len <= vma->vm_start && addr > mm->brk);
 
 	/*
 	 * A failed mmap() very likely causes application failure,
--
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