hugetlb: fix pool resizing corner case

When shrinking the size of the hugetlb pool via the nr_hugepages sysctl, we
are careful to keep enough pages around to satisfy reservations.  But the
calculation is flawed for the following scenario:

Action                          Pool Counters (Total, Free, Resv)
======                          =============
Set pool to 1 page              1 1 0
Map 1 page MAP_PRIVATE          1 1 0
Touch the page to fault it in   1 0 0
Set pool to 3 pages             3 2 0
Map 2 pages MAP_SHARED          3 2 2
Set pool to 2 pages             2 1 2 <-- Mistake, should be 3 2 2
Touch the 2 shared pages        2 0 1 <-- Program crashes here

The last touch above will terminate the process due to lack of huge pages.

This patch corrects the calculation so that it factors in pages being used
for private mappings.  Andrew, this is a standalone fix suitable for
mainline.  It is also now corrected in my latest dynamic pool resizing
patchset which I will send out soon.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Acked-by: Ken Chen <kenchen@google.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: William Lee Irwin III <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -403,14 +403,14 @@ static void try_to_free_low(unsigned long count)
 	for (i = 0; i < MAX_NUMNODES; ++i) {
 		struct page *page, *next;
 		list_for_each_entry_safe(page, next, &hugepage_freelists[i], lru) {
+			if (count >= nr_huge_pages)
+				return;
 			if (PageHighMem(page))
 				continue;
 			list_del(&page->lru);
 			update_and_free_page(page);
 			free_huge_pages--;
 			free_huge_pages_node[page_to_nid(page)]--;
-			if (count >= nr_huge_pages)
-				return;
 		}
 	}
 }
@@ -450,8 +450,6 @@ static unsigned long set_max_huge_pages(unsigned long count)
 			goto out;
 
 	}
-	if (count >= persistent_huge_pages)
-		goto out;
 
 	/*
 	 * Decrease the pool size
@@ -460,7 +458,8 @@ static unsigned long set_max_huge_pages(unsigned long count)
 	 * pages into surplus state as needed so the pool will shrink
 	 * to the desired size as pages become free.
 	 */
-	min_count = max(count, resv_huge_pages);
+	min_count = resv_huge_pages + nr_huge_pages - free_huge_pages;
+	min_count = max(count, min_count);
 	try_to_free_low(min_count);
 	while (min_count < persistent_huge_pages) {
 		struct page *page = dequeue_huge_page(NULL, 0);