[RFC PATCH 1/8] hugetlb: add per-hstate mutex to synchronize user adjustments

[Mike Kravetz]

The number of hugetlb pages can be adjusted by writing to the
sysps/proc files nr_hugepages, nr_hugepages_mempolicy or
nr_overcommit_hugepages.  There is nothing to prevent two
concurrent modifications via these files.  The underlying routine
set_max_huge_pages() makes assumptions that only one occurrence is
running at a time.  Specifically, alloc_pool_huge_page uses a
hstate specific variable without any synchronization.

Add a mutex to the hstate and use it to only allow one hugetlb
page adjustment at a time.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 include/linux/hugetlb.h | 1 +
 mm/hugetlb.c            | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index cccd1aab69dd..f42d44050548 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -555,6 +555,7 @@ HPAGEFLAG(Freed, freed)
 #define HSTATE_NAME_LEN 32
 /* Defines one hugetlb page size */
 struct hstate {
+	struct mutex mutex;
 	int next_nid_to_alloc;
 	int next_nid_to_free;
 	unsigned int order;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5b1ab1f427c5..d5be25f910e8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2601,6 +2601,8 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
 	else
 		return -ENOMEM;
 
+	/* mutex prevents concurrent adjustments for the same hstate */
+	mutex_lock(&h->mutex);
 	spin_lock(&hugetlb_lock);
 
 	/*
@@ -2633,6 +2635,7 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
 	if (hstate_is_gigantic(h) && !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
 		if (count > persistent_huge_pages(h)) {
 			spin_unlock(&hugetlb_lock);
+			mutex_unlock(&h->mutex);
 			NODEMASK_FREE(node_alloc_noretry);
 			return -EINVAL;
 		}
@@ -2707,6 +2710,7 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
 out:
 	h->max_huge_pages = persistent_huge_pages(h);
 	spin_unlock(&hugetlb_lock);
+	mutex_unlock(&h->mutex);
 
 	NODEMASK_FREE(node_alloc_noretry);
 
@@ -3194,6 +3198,7 @@ void __init hugetlb_add_hstate(unsigned int order)
 	BUG_ON(hugetlb_max_hstate >= HUGE_MAX_HSTATE);
 	BUG_ON(order == 0);
 	h = &hstates[hugetlb_max_hstate++];
+	mutex_init(&h->mutex);
 	h->order = order;
 	h->mask = ~(huge_page_size(h) - 1);
 	for (i = 0; i < MAX_NUMNODES; ++i)
-- 
2.30.2