[PATCH 04/21] mm/hugetlb: refactor alloc_and_dissolve_huge_page

[Doug Berger <opendmb-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

The alloc_replacement_page() and replace_hugepage() functions are
created from code in the alloc_and_dissolve_huge_page() function
to allow their reuse by the next commit.

Signed-off-by: Doug Berger <opendmb-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
---
 mm/hugetlb.c | 84 +++++++++++++++++++++++++++++++---------------------
 1 file changed, 51 insertions(+), 33 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a1d51a1f0404..f232a37df4b6 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2709,32 +2709,22 @@ void restore_reserve_on_error(struct hstate *h, struct vm_area_struct *vma,
 }
 
 /*
- * alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
- * @h: struct hstate old page belongs to
- * @old_page: Old page to dissolve
- * @list: List to isolate the page in case we need to
- * Returns 0 on success, otherwise negated error.
+ * Before dissolving the page, we need to allocate a new one for the
+ * pool to remain stable.  Here, we allocate the page and 'prep' it
+ * by doing everything but actually updating counters and adding to
+ * the pool.  This simplifies and let us do most of the processing
+ * under the lock.
  */
-static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
-					struct list_head *list)
+static struct page *alloc_replacement_page(struct hstate *h, int nid)
 {
 	gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
-	int nid = page_to_nid(old_page);
 	bool alloc_retry = false;
 	struct page *new_page;
-	int ret = 0;
 
-	/*
-	 * Before dissolving the page, we need to allocate a new one for the
-	 * pool to remain stable.  Here, we allocate the page and 'prep' it
-	 * by doing everything but actually updating counters and adding to
-	 * the pool.  This simplifies and let us do most of the processing
-	 * under the lock.
-	 */
 alloc_retry:
 	new_page = alloc_buddy_huge_page(h, gfp_mask, nid, NULL, NULL);
 	if (!new_page)
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 	/*
 	 * If all goes well, this page will be directly added to the free
 	 * list in the pool.  For this the ref count needs to be zero.
@@ -2748,7 +2738,7 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 	SetHPageTemporary(new_page);
 	if (!put_page_testzero(new_page)) {
 		if (alloc_retry)
-			return -EBUSY;
+			return ERR_PTR(-EBUSY);
 
 		alloc_retry = true;
 		goto alloc_retry;
@@ -2757,6 +2747,48 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 
 	__prep_new_huge_page(h, new_page);
 
+	return new_page;
+}
+
+static void replace_hugepage(struct hstate *h, int nid, struct page *old_page,
+			     struct page *new_page)
+{
+	lockdep_assert_held(&hugetlb_lock);
+	/*
+	 * Ok, old_page is still a genuine free hugepage. Remove it from
+	 * the freelist and decrease the counters. These will be
+	 * incremented again when calling __prep_account_new_huge_page()
+	 * and enqueue_huge_page() for new_page. The counters will remain
+	 * stable since this happens under the lock.
+	 */
+	remove_hugetlb_page(h, old_page, false);
+
+	/*
+	 * Ref count on new page is already zero as it was dropped
+	 * earlier.  It can be directly added to the pool free list.
+	 */
+	__prep_account_new_huge_page(h, nid);
+	enqueue_huge_page(h, new_page);
+}
+
+/*
+ * alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
+ * @h: struct hstate old page belongs to
+ * @old_page: Old page to dissolve
+ * @list: List to isolate the page in case we need to
+ * Returns 0 on success, otherwise negated error.
+ */
+static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
+					struct list_head *list)
+{
+	int nid = page_to_nid(old_page);
+	struct page *new_page;
+	int ret = 0;
+
+	new_page = alloc_replacement_page(h, nid);
+	if (IS_ERR(new_page))
+		return PTR_ERR(new_page);
+
 retry:
 	spin_lock_irq(&hugetlb_lock);
 	if (!PageHuge(old_page)) {
@@ -2783,21 +2815,7 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 		cond_resched();
 		goto retry;
 	} else {
-		/*
-		 * Ok, old_page is still a genuine free hugepage. Remove it from
-		 * the freelist and decrease the counters. These will be
-		 * incremented again when calling __prep_account_new_huge_page()
-		 * and enqueue_huge_page() for new_page. The counters will remain
-		 * stable since this happens under the lock.
-		 */
-		remove_hugetlb_page(h, old_page, false);
-
-		/*
-		 * Ref count on new page is already zero as it was dropped
-		 * earlier.  It can be directly added to the pool free list.
-		 */
-		__prep_account_new_huge_page(h, nid);
-		enqueue_huge_page(h, new_page);
+		replace_hugepage(h, nid, old_page, new_page);
 
 		/*
 		 * Pages have been replaced, we can safely free the old one.
-- 
2.25.1