[PATCH 3/8] hugetlb: add per-hstate mutex to synchronize user adjustments

[Mike Kravetz <mike.kravetz@xxxxxxxxxx>]

The helper routine hstate_next_node_to_alloc accesses and modifies the
hstate variable next_nid_to_alloc.  The helper is used by the routines
alloc_pool_huge_page and adjust_pool_surplus.  adjust_pool_surplus is
called with hugetlb_lock held.  However, alloc_pool_huge_page can not
be called with the hugetlb lock held as it will call the page allocator.
Two instances of alloc_pool_huge_page could be run in parallel or
alloc_pool_huge_page could run in parallel with adjust_pool_surplus
which may result in the variable next_nid_to_alloc becoming invalid
for the caller and pages being allocated on the wrong node.

Both alloc_pool_huge_page and adjust_pool_surplus are only called from
the routine set_max_huge_pages after boot.  set_max_huge_pages is only
called as the reusult of a user writing to the proc/sysfs nr_hugepages,
or nr_hugepages_mempolicy file to adjust the number of hugetlb pages.

It makes little sense to allow multiple adjustment to the number of
hugetlb pages in parallel.  Add a mutex to the hstate and use it to only
allow one hugetlb page adjustment at a time.  This will synchronize
modifications to the next_nid_to_alloc variable.

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 a7f7d5f328dc..8817ec987d68 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -566,6 +566,7 @@ 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 f9ba63fc1747..404b0b1c5258 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2616,6 +2616,8 @@ 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);
 
 	/*
@@ -2648,6 +2650,7 @@ 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;
 		}
@@ -2722,6 +2725,7 @@ 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);
 
@@ -3209,6 +3213,7 @@ 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