hugetlb: add locking for overcommit sysctl

When I replaced hugetlb_dynamic_pool with nr_overcommit_hugepages I used
proc_doulongvec_minmax() directly.  However, hugetlb.c's locking rules
require that all counter modifications occur under the hugetlb_lock.  Add a
callback into the hugetlb code similar to the one for nr_hugepages.  Grab
the lock around the manipulation of nr_overcommit_hugepages in
proc_doulongvec_minmax().

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Acked-by: Adam Litke <agl@us.ibm.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/hugetlb.h

@@ -17,6 +17,7 @@ static inline int is_vm_hugetlb_page(struct vm_area_struct *vma)
 }
 
 int hugetlb_sysctl_handler(struct ctl_table *, int, struct file *, void __user *, size_t *, loff_t *);
+int hugetlb_overcommit_handler(struct ctl_table *, int, struct file *, void __user *, size_t *, loff_t *);
 int hugetlb_treat_movable_handler(struct ctl_table *, int, struct file *, void __user *, size_t *, loff_t *);
 int copy_hugetlb_page_range(struct mm_struct *, struct mm_struct *, struct vm_area_struct *);
 int follow_hugetlb_page(struct mm_struct *, struct vm_area_struct *, struct page **, struct vm_area_struct **, unsigned long *, int *, int, int);

kernel/sysctl.c

@@ -982,7 +982,7 @@ static struct ctl_table vm_table[] = {
 		.data		= &nr_overcommit_huge_pages,
 		.maxlen		= sizeof(nr_overcommit_huge_pages),
 		.mode		= 0644,
-		.proc_handler	= &proc_doulongvec_minmax,
+		.proc_handler	= &hugetlb_overcommit_handler,
 	},
 #endif
 	{

mm/hugetlb.c

@@ -605,6 +605,16 @@ int hugetlb_treat_movable_handler(struct ctl_table *table, int write,
 	return 0;
 }
 
+int hugetlb_overcommit_handler(struct ctl_table *table, int write,
+			struct file *file, void __user *buffer,
+			size_t *length, loff_t *ppos)
+{
+	spin_lock(&hugetlb_lock);
+	proc_doulongvec_minmax(table, write, file, buffer, length, ppos);
+	spin_unlock(&hugetlb_lock);
+	return 0;
+}
+
 #endif /* CONFIG_SYSCTL */
 
 int hugetlb_report_meminfo(char *buf)