[PATCH 1/2] hugetlb: search harder for memory in alloc_fresh_huge_page()

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

Currently, alloc_fresh_huge_page() returns NULL when it is not able to
allocate a huge page on the current node, as specified by its custom
interleave variable. The callers of this function, though, assume that a
failure in alloc_fresh_huge_page() indicates no hugepages can be
allocated on the system period. This might not be the case, for
instance, if we have an uneven NUMA system, and we happen to try to
allocate a hugepage on a node with less memory and fail, while there is
still plenty of free memory on the other nodes.

To correct this, make alloc_fresh_huge_page() search through all online
nodes before deciding no hugepages can be allocated. Add a helper
function for actually allocating the hugepage.

Note: we expect particular semantics for __GFP_THISNODE, which are now
enforced even for memoryless nodes. That is, there is should be no
fallback to other nodes. Therefore, we rely on the nid passed into
alloc_pages_node() to be the nid the page comes from. If this is
incorrect, accounting will break.

Tested on x86 !NUMA, x86 NUMA, x86_64 NUMA and ppc64 NUMA (with 2
memoryless nodes).

Before on the ppc64 box:
Trying to clear the hugetlb pool
Done.       0 free
Trying to resize the pool to 100
Node 0 HugePages_Free:     25
Node 1 HugePages_Free:     75
Node 2 HugePages_Free:      0
Node 3 HugePages_Free:      0
Done. Initially     100 free
Trying to resize the pool to 200
Node 0 HugePages_Free:     50
Node 1 HugePages_Free:    150
Node 2 HugePages_Free:      0
Node 3 HugePages_Free:      0
Done.     200 free

After:
Trying to clear the hugetlb pool
Done.       0 free
Trying to resize the pool to 100
Node 0 HugePages_Free:     50
Node 1 HugePages_Free:     50
Node 2 HugePages_Free:      0
Node 3 HugePages_Free:      0
Done. Initially     100 free
Trying to resize the pool to 200
Node 0 HugePages_Free:    100
Node 1 HugePages_Free:    100
Node 2 HugePages_Free:      0
Node 3 HugePages_Free:      0
Done.     200 free

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>

---
Christoph, I've moved to using a global static variable, is this closer
to what you hoped for?

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4a374fa..d97508e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -29,6 +29,7 @@ static unsigned int nr_huge_pages_node[MAX_NUMNODES];
 static unsigned int free_huge_pages_node[MAX_NUMNODES];
 static gfp_t htlb_alloc_mask = GFP_HIGHUSER;
 unsigned long hugepages_treat_as_movable;
+static int last_allocated_nid;
 
 /*
  * Protects updates to hugepage_freelists, nr_huge_pages, and free_huge_pages
@@ -103,36 +104,56 @@ static void free_huge_page(struct page *page)
 	spin_unlock(&hugetlb_lock);
 }
 
-static int alloc_fresh_huge_page(void)
+static struct page *alloc_fresh_huge_page_node(int nid)
 {
-	static int prev_nid;
 	struct page *page;
-	int nid;
-
-	/*
-	 * Copy static prev_nid to local nid, work on that, then copy it
-	 * back to prev_nid afterwards: otherwise there's a window in which
-	 * a racer might pass invalid nid MAX_NUMNODES to alloc_pages_node.
-	 * But we don't need to use a spin_lock here: it really doesn't
-	 * matter if occasionally a racer chooses the same nid as we do.
-	 */
-	nid = next_node(prev_nid, node_online_map);
-	if (nid == MAX_NUMNODES)
-		nid = first_node(node_online_map);
-	prev_nid = nid;
 
-	page = alloc_pages_node(nid, htlb_alloc_mask|__GFP_COMP|__GFP_NOWARN,
-					HUGETLB_PAGE_ORDER);
+	page = alloc_pages_node(nid,
+		htlb_alloc_mask|__GFP_COMP|__GFP_THISNODE|__GFP_NOWARN,
+		HUGETLB_PAGE_ORDER);
 	if (page) {
 		set_compound_page_dtor(page, free_huge_page);
 		spin_lock(&hugetlb_lock);
 		nr_huge_pages++;
-		nr_huge_pages_node[page_to_nid(page)]++;
+		nr_huge_pages_node[nid]++;
 		spin_unlock(&hugetlb_lock);
 		put_page(page); /* free it into the hugepage allocator */
-		return 1;
 	}
-	return 0;
+
+	return page;
+}
+
+static int alloc_fresh_huge_page(void)
+{
+	struct page *page;
+	int start_nid;
+	int next_nid;
+	int ret = 0;
+
+	start_nid = last_allocated_nid;
+
+	do {
+		page = alloc_fresh_huge_page_node(last_allocated_nid);
+		if (page)
+			ret = 1;
+		/*
+		 * Use a helper variable to find the next node and then
+		 * copy it back to last_allocated_nid afterwards:
+		 * otherwise there's a window in which a racer might
+		 * pass invalid nid MAX_NUMNODES to alloc_pages_node.
+		 * But we don't need to use a spin_lock here: it really
+		 * doesn't matter if occasionally a racer chooses the
+		 * same nid as we do.  Move nid forward in the mask even
+		 * if we just successfully allocated a hugepage so that
+		 * the next caller gets hugepages on the next node.
+		 */
+		next_nid = next_node(last_allocated_nid, node_online_map);
+		if (next_nid == MAX_NUMNODES)
+			next_nid = first_node(node_online_map);
+		last_allocated_nid = next_nid;
+	} while (!page && last_allocated_nid != start_nid);
+
+	return ret;
 }
 
 static struct page *alloc_huge_page(struct vm_area_struct *vma,
@@ -171,6 +192,8 @@ static int __init hugetlb_init(void)
 	for (i = 0; i < MAX_NUMNODES; ++i)
 		INIT_LIST_HEAD(&hugepage_freelists[i]);
 
+	last_allocated_nid = first_node(node_online_map);
+
 	for (i = 0; i < max_huge_pages; ++i) {
 		if (!alloc_fresh_huge_page())
 			break;

-- 
Nishanth Aravamudan <nacc@us.ibm.com>
IBM Linux Technology Center

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