mm/hugetlb: fix nodes huge page allocation when there are surplus pages

In set_nr_huge_pages(), local variable "count" is used to record
persistent_huge_pages(), but when it cames to nodes huge page allocation,
the semantics changes to nr_huge_pages.  When there exists surplus huge
pages and using the interface under
/sys/devices/system/node/node*/hugepages to change huge page pool size,
this difference can result in the allocation of an unexpected number of
huge pages.

Steps to reproduce the bug:

Starting with:

				  Node 0          Node 1    Total
	HugePages_Total             0.00            0.00     0.00
	HugePages_Free              0.00            0.00     0.00
	HugePages_Surp              0.00            0.00     0.00

create 100 huge pages in Node 0 and consume it, then set Node 0 's
nr_hugepages to 0.

yields:

				  Node 0          Node 1    Total
	HugePages_Total           200.00            0.00   200.00
	HugePages_Free              0.00            0.00     0.00
	HugePages_Surp            200.00            0.00   200.00

write 100 to Node 1's nr_hugepages

		echo 100 > /sys/devices/system/node/node1/\
	hugepages/hugepages-2048kB/nr_hugepages

gets:

				  Node 0          Node 1    Total
	HugePages_Total           200.00          400.00   600.00
	HugePages_Free              0.00          400.00   400.00
	HugePages_Surp            200.00            0.00   200.00

Kernel is expected to create only 100 huge pages and it gives 200.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 9a30523 ("hugetlb: add per node hstate attributes")
Signed-off-by: Xueshi Hu <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Martins3

mm/hugetlb.c

@@ -3457,7 +3457,9 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
 	if (nid != NUMA_NO_NODE) {
 		unsigned long old_count = count;
 
-		count += h->nr_huge_pages - h->nr_huge_pages_node[nid];
+		count += persistent_huge_pages(h) -
+			 (h->nr_huge_pages_node[nid] -
+			  h->surplus_huge_pages_node[nid]);
 		/*
 		 * User may have specified a large count value which caused the
 		 * above calculation to overflow.  In this case, they wanted