Btrfs: Use an array of pages in the extent buffers to reduce the cost of find_get_page

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -647,20 +647,20 @@ int close_ctree(struct btrfs_root *root)
 
 int btrfs_buffer_uptodate(struct extent_buffer *buf)
 {
-	struct inode *btree_inode = buf->first_page->mapping->host;
+	struct inode *btree_inode = buf->pages[0]->mapping->host;
 	return extent_buffer_uptodate(&BTRFS_I(btree_inode)->extent_tree, buf);
 }
 
 int btrfs_set_buffer_uptodate(struct extent_buffer *buf)
 {
-	struct inode *btree_inode = buf->first_page->mapping->host;
+	struct inode *btree_inode = buf->pages[0]->mapping->host;
 	return set_extent_buffer_uptodate(&BTRFS_I(btree_inode)->extent_tree,
 					  buf);
 }
 
 void btrfs_mark_buffer_dirty(struct extent_buffer *buf)
 {
-	struct btrfs_root *root = BTRFS_I(buf->first_page->mapping->host)->root;
+	struct btrfs_root *root = BTRFS_I(buf->pages[0]->mapping->host)->root;
 	u64 transid = btrfs_header_generation(buf);
 	struct inode *btree_inode = root->fs_info->btree_inode;
 

fs/btrfs/extent_map.c

@@ -1961,16 +1961,12 @@ static void __free_extent_buffer(struct extent_buffer *eb)
 static inline struct page *extent_buffer_page(struct extent_buffer *eb, int i)
 {
 	struct page *p;
-	if (i == 0)
-		return eb->first_page;
 
+	if (i < EXTENT_INLINE_PAGES)
+		return eb->pages[i];
 	i += eb->start >> PAGE_CACHE_SHIFT;
-	if (eb->last_page && eb->last_page->index == i)
-		return eb->last_page;
-
-	p = find_get_page(eb->first_page->mapping, i);
+	p = find_get_page(eb->pages[0]->mapping, i);
 	page_cache_release(p);
-	eb->last_page = p;
 	return p;
 }
 
@@ -2012,8 +2008,8 @@ struct extent_buffer *alloc_extent_buffer(struct extent_map_tree *tree,
 			goto fail;
 		}
 		set_page_extent_mapped(p);
-		if (i == 0)
-			eb->first_page = p;
+		if (i < EXTENT_INLINE_PAGES)
+			eb->pages[i] = p;
 		if (!PageUptodate(p))
 			uptodate = 0;
 		unlock_page(p);
@@ -2059,8 +2055,8 @@ struct extent_buffer *find_extent_buffer(struct extent_map_tree *tree,
 			goto fail;
 		}
 		set_page_extent_mapped(p);
-		if (i == 0)
-			eb->first_page = p;
+		if (i < EXTENT_INLINE_PAGES)
+			eb->pages[i] = p;
 		if (!PageUptodate(p))
 			uptodate = 0;
 		unlock_page(p);
@@ -2087,9 +2083,7 @@ void free_extent_buffer(struct extent_buffer *eb)
 
 	num_pages = num_extent_pages(eb->start, eb->len);
 
-	if (eb->first_page)
-		page_cache_release(eb->first_page);
-	for (i = 1; i < num_pages; i++) {
+	for (i = 0; i < num_pages; i++) {
 		page_cache_release(extent_buffer_page(eb, i));
 	}
 	__free_extent_buffer(eb);

fs/btrfs/extent_map.h

@@ -62,6 +62,7 @@ struct extent_state {
 	struct list_head list;
 };
 
+#define EXTENT_INLINE_PAGES 32
 struct extent_buffer {
 	u64 start;
 	unsigned long len;
@@ -69,13 +70,12 @@ struct extent_buffer {
 	int flags;
 	struct list_head list;
 	struct list_head leak_list;
-	struct page *first_page;
-	struct page *last_page;
 	unsigned long alloc_addr;
 	char *map_token;
 	char *kaddr;
 	unsigned long map_start;
 	unsigned long map_len;
+	struct page *pages[EXTENT_INLINE_PAGES];
 };
 
 typedef struct extent_map *(get_extent_t)(struct inode *inode,