[PATCH v2 6/8] hugetlb: change free_pool_huge_page to remove_pool_huge_page

[Mike Kravetz]

free_pool_huge_page was called with hugetlb_lock held.  It would remove
a hugetlb page, and then free the corresponding pages to the lower level
allocators such as buddy.  free_pool_huge_page was called in a loop to
remove hugetlb pages and these loops could hold the hugetlb_lock for a
considerable time.

Create new routine remove_pool_huge_page to replace free_pool_huge_page.
remove_pool_huge_page will remove the hugetlb page, and it must be
called with the hugetlb_lock held.  It will return the removed page and
it is the responsibility of the caller to free the page to the lower
level allocators.  The hugetlb_lock is dropped before freeing to these
allocators which results in shorter lock hold times.

Add new helper routine to call update_and_free_page for a list of pages.

Note: Some changes to the routine return_unused_surplus_pages are in
need of explanation.  Commit e5bbc8a6c992 ("mm/hugetlb.c: fix reservation
race when freeing surplus pages") modified this routine to address a
race which could occur when dropping the hugetlb_lock in the loop that
removes pool pages.  Accounting changes introduced in that commit were
subtle and took some thought to understand.  This commit removes the
cond_resched_lock() and the potential race.  Therefore, remove the
subtle code and restore the more straight forward accounting effectively
reverting the commit.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 95 +++++++++++++++++++++++++++++-----------------------
 1 file changed, 53 insertions(+), 42 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index dec7bd0dc63d..d3f3cb8766b8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1209,7 +1209,7 @@ static int hstate_next_node_to_alloc(struct hstate *h,
 }
 
 /*
- * helper for free_pool_huge_page() - return the previously saved
+ * helper for remove_pool_huge_page() - return the previously saved
  * node ["this node"] from which to free a huge page.  Advance the
  * next node id whether or not we find a free huge page to free so
  * that the next attempt to free addresses the next node.
@@ -1391,6 +1391,16 @@ static void update_and_free_page(struct hstate *h, struct page *page)
 	}
 }
 
+static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
+{
+	struct page *page, *t_page;
+
+	list_for_each_entry_safe(page, t_page, list, lru) {
+		update_and_free_page(h, page);
+		cond_resched();
+	}
+}
+
 struct hstate *size_to_hstate(unsigned long size)
 {
 	struct hstate *h;
@@ -1721,16 +1731,18 @@ static int alloc_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
 }
 
 /*
- * Free huge page from pool from next node to free.
- * Attempt to keep persistent huge pages more or less
- * balanced over allowed nodes.
+ * Remove huge page from pool from next node to free.  Attempt to keep
+ * persistent huge pages more or less balanced over allowed nodes.
+ * This routine only 'removes' the hugetlb page.  The caller must make
+ * an additional call to free the page to low level allocators.
  * Called with hugetlb_lock locked.
  */
-static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
-							 bool acct_surplus)
+static struct page *remove_pool_huge_page(struct hstate *h,
+						nodemask_t *nodes_allowed,
+						 bool acct_surplus)
 {
 	int nr_nodes, node;
-	int ret = 0;
+	struct page *page = NULL;
 
 	for_each_node_mask_to_free(h, nr_nodes, node, nodes_allowed) {
 		/*
@@ -1739,23 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
 		 */
 		if ((!acct_surplus || h->surplus_huge_pages_node[node]) &&
 		    !list_empty(&h->hugepage_freelists[node])) {
-			struct page *page =
-				list_entry(h->hugepage_freelists[node].next,
+			page = list_entry(h->hugepage_freelists[node].next,
 					  struct page, lru);
 			remove_hugetlb_page(h, page, acct_surplus);
-			/*
-			 * unlock/lock around update_and_free_page is temporary
-			 * and will be removed with subsequent patch.
-			 */
-			spin_unlock(&hugetlb_lock);
-			update_and_free_page(h, page);
-			spin_lock(&hugetlb_lock);
-			ret = 1;
 			break;
 		}
 	}
 
-	return ret;
+	return page;
 }
 
 /*
@@ -2075,17 +2078,16 @@ static int gather_surplus_pages(struct hstate *h, long delta)
  *    to the associated reservation map.
  * 2) Free any unused surplus pages that may have been allocated to satisfy
  *    the reservation.  As many as unused_resv_pages may be freed.
- *
- * Called with hugetlb_lock held.  However, the lock could be dropped (and
- * reacquired) during calls to cond_resched_lock.  Whenever dropping the lock,
- * we must make sure nobody else can claim pages we are in the process of
- * freeing.  Do this by ensuring resv_huge_page always is greater than the
- * number of huge pages we plan to free when dropping the lock.
  */
 static void return_unused_surplus_pages(struct hstate *h,
 					unsigned long unused_resv_pages)
 {
 	unsigned long nr_pages;
+	struct page *page;
+	LIST_HEAD(page_list);
+
+	/* Uncommit the reservation */
+	h->resv_huge_pages -= unused_resv_pages;
 
 	/* Cannot return gigantic pages currently */
 	if (hstate_is_gigantic(h))
@@ -2102,24 +2104,22 @@ static void return_unused_surplus_pages(struct hstate *h,
 	 * evenly across all nodes with memory. Iterate across these nodes
 	 * until we can no longer free unreserved surplus pages. This occurs
 	 * when the nodes with surplus pages have no free pages.
-	 * free_pool_huge_page() will balance the freed pages across the
+	 * remove_pool_huge_page() will balance the freed pages across the
 	 * on-line nodes with memory and will handle the hstate accounting.
-	 *
-	 * Note that we decrement resv_huge_pages as we free the pages.  If
-	 * we drop the lock, resv_huge_pages will still be sufficiently large
-	 * to cover subsequent pages we may free.
 	 */
+	INIT_LIST_HEAD(&page_list);
 	while (nr_pages--) {
-		h->resv_huge_pages--;
-		unused_resv_pages--;
-		if (!free_pool_huge_page(h, &node_states[N_MEMORY], 1))
+		page = remove_pool_huge_page(h, &node_states[N_MEMORY], 1);
+		if (!page)
 			goto out;
-		cond_resched_lock(&hugetlb_lock);
+
+		list_add(&page->lru, &page_list);
 	}
 
 out:
-	/* Fully uncommit the reservation */
-	h->resv_huge_pages -= unused_resv_pages;
+	spin_unlock(&hugetlb_lock);
+	update_and_free_pages_bulk(h, &page_list);
+	spin_lock(&hugetlb_lock);
 }
 
 
@@ -2683,7 +2683,6 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
 						nodemask_t *nodes_allowed)
 {
 	int i;
-	struct page *page, *next;
 	LIST_HEAD(page_list);
 
 	if (hstate_is_gigantic(h))
@@ -2694,6 +2693,7 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
 	 */
 	INIT_LIST_HEAD(&page_list);
 	for_each_node_mask(i, *nodes_allowed) {
+		struct page *page, *next;
 		struct list_head *freel = &h->hugepage_freelists[i];
 		list_for_each_entry_safe(page, next, freel, lru) {
 			if (count >= h->nr_huge_pages)
@@ -2707,10 +2707,7 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
 
 out:
 	spin_unlock(&hugetlb_lock);
-	list_for_each_entry_safe(page, next, &page_list, lru) {
-		update_and_free_page(h, page);
-		cond_resched();
-	}
+	update_and_free_pages_bulk(h, &page_list);
 	spin_lock(&hugetlb_lock);
 }
 #else
@@ -2757,6 +2754,8 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
 			      nodemask_t *nodes_allowed)
 {
 	unsigned long min_count, ret;
+	struct page *page;
+	LIST_HEAD(page_list);
 	NODEMASK_ALLOC(nodemask_t, node_alloc_noretry, GFP_KERNEL);
 
 	/*
@@ -2869,11 +2868,23 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
 	min_count = h->resv_huge_pages + h->nr_huge_pages - h->free_huge_pages;
 	min_count = max(count, min_count);
 	try_to_free_low(h, min_count, nodes_allowed);
+
+	/*
+	 * Collect pages to be removed on list without dropping lock
+	 */
+	INIT_LIST_HEAD(&page_list);
 	while (min_count < persistent_huge_pages(h)) {
-		if (!free_pool_huge_page(h, nodes_allowed, 0))
+		page = remove_pool_huge_page(h, nodes_allowed, 0);
+		if (!page)
 			break;
-		cond_resched_lock(&hugetlb_lock);
+
+		list_add(&page->lru, &page_list);
 	}
+	/* free the pages after dropping lock */
+	spin_unlock(&hugetlb_lock);
+	update_and_free_pages_bulk(h, &page_list);
+	spin_lock(&hugetlb_lock);
+
 	while (count < persistent_huge_pages(h)) {
 		if (!adjust_pool_surplus(h, nodes_allowed, 1))
 			break;
-- 
2.30.2