[RFC PATCH 1/2] mm: Batch unmapping of file mapped pages in shrink_page_list

[RFC PATCH 1/2] mm: Batch unmapping of file mapped pages in shrink_page_list

[Tim Chen]

We gather the pages that need to be unmapped in shrink_page_list.  We do
the unmap in a single batch to reduce the frequency of acquisition of
the tree lock protecting the address mapping radix tree. This is
possible as successive pages likely share the same mapping in the
__remove_mapping_batch routine.  This avoids excessive cache bouncing of
the tree lock when page reclamations are occurring simultaneously.

Tim

---
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
diff --git a/mm/vmscan.c b/mm/vmscan.c
index aac5672..a538565 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -600,6 +600,85 @@ cannot_free:
 	return 0;
 }
 
+/* Same as __remove_mapping, but batching operations to minimize locking */
+/* Pages to be unmapped should be locked first */
+static int __remove_mapping_batch(struct list_head *unmap_pages,
+				  struct list_head *ret_pages,
+				  struct list_head *free_pages)
+{
+	struct address_space *mapping, *next;
+	LIST_HEAD(swap_pages);
+	swp_entry_t swap;
+	struct page *page;
+	int nr_reclaimed;
+
+	mapping = NULL;
+	nr_reclaimed = 0;
+	while (!list_empty(unmap_pages)) {
+
+		page = lru_to_page(unmap_pages);
+		BUG_ON(!PageLocked(page));
+
+		list_del(&page->lru);
+		next = page_mapping(page);
+		if (mapping != next) {
+			if (mapping)
+				spin_unlock_irq(&mapping->tree_lock);
+			mapping = next;
+			spin_lock_irq(&mapping->tree_lock);
+		}
+
+		if (!page_freeze_refs(page, 2))
+			goto cannot_free;
+		if (unlikely(PageDirty(page))) {
+			page_unfreeze_refs(page, 2);
+			goto cannot_free;
+		}
+
+		if (PageSwapCache(page)) {
+			__delete_from_swap_cache(page);
+			/* swapcahce_free need to be called without tree_lock */
+			list_add(&page->lru, &swap_pages);
+		} else {
+			void (*freepage)(struct page *);
+
+			freepage = mapping->a_ops->freepage;
+
+			__delete_from_page_cache(page);
+			mem_cgroup_uncharge_cache_page(page);
+
+			if (freepage != NULL)
+				freepage(page);
+
+			unlock_page(page);
+			nr_reclaimed++;
+			list_add(&page->lru, free_pages);
+		}
+		continue;
+cannot_free:
+		unlock_page(page);
+		list_add(&page->lru, ret_pages);
+		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
+
+	}
+
+	if (mapping)
+		spin_unlock_irq(&mapping->tree_lock);
+
+	while (!list_empty(&swap_pages)) {
+		page = lru_to_page(&swap_pages);
+		list_del(&page->lru);
+
+		swap.val = page_private(page);
+		swapcache_free(swap, page);
+
+		unlock_page(page);
+		nr_reclaimed++;
+		list_add(&page->lru, free_pages);
+	}
+
+	return nr_reclaimed;
+}
 /*
  * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
  * someone else has a ref on the page, abort and return 0.  If it was
@@ -771,6 +850,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 {
 	LIST_HEAD(ret_pages);
 	LIST_HEAD(free_pages);
+	LIST_HEAD(unmap_pages);
 	int pgactivate = 0;
 	unsigned long nr_dirty = 0;
 	unsigned long nr_congested = 0;
@@ -969,17 +1049,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			}
 		}
 
-		if (!mapping || !__remove_mapping(mapping, page))
+		if (!mapping)
 			goto keep_locked;
 
-		/*
-		 * At this point, we have no other references and there is
-		 * no way to pick any more up (removed from LRU, removed
-		 * from pagecache). Can use non-atomic bitops now (and
-		 * we obviously don't have to worry about waking up a process
-		 * waiting on the page lock, because there are no references.
-		 */
-		__clear_page_locked(page);
+		/* remove pages from mapping in batch at end of loop */
+		list_add(&page->lru, &unmap_pages);
+		continue;
+
 free_it:
 		nr_reclaimed++;
 
@@ -1014,6 +1090,9 @@ keep_lumpy:
 		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
 	}
 
+	nr_reclaimed += __remove_mapping_batch(&unmap_pages, &ret_pages,
+					       &free_pages);
+
 	/*
 	 * Tag a zone as congested if all the dirty pages encountered were
 	 * backed by a congested BDI. In this case, reclaimers should just


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