[PATCH] hugetlb: add locking for overcommit sysctl

[Nishanth Aravamudan <nacc@us.ibm.com>]

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>

---
I noticed that the nr_hugepages sysctl uses a helper variable,
max_huge_pages to do the same thing. Is that because of locking
requirements (undocmented) for proc_doulongvec_minmax() or because of
the more complicated manipulation of the pool state for that sysctl? If
the former, I will need to modify this patch.

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 30d606a..7ca198b 100644
--- a/include/linux/hugetlb.h
+++ b/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);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 5e2ad5b..92fef8d 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -973,7 +973,7 @@ 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
 	{
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1a56420..d9a3803 100644
--- a/mm/hugetlb.c
+++ b/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)

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