f2fs: introduce nr_pages_to_write for segment alignment

This patch introduces nr_pages_to_write to align page writes to the segment
or other operational unit size, which can be tuned according to the system
environment.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/checkpoint.c

@@ -187,18 +187,19 @@ static int f2fs_write_meta_pages(struct address_space *mapping,
 				struct writeback_control *wbc)
 {
 	struct f2fs_sb_info *sbi = F2FS_SB(mapping->host->i_sb);
-	int nrpages = nr_pages_to_skip(sbi, META);
-	long written;
+	long diff, written;
 
 	/* collect a number of dirty meta pages and write together */
-	if (wbc->for_kupdate || get_pages(sbi, F2FS_DIRTY_META) < nrpages)
+	if (wbc->for_kupdate ||
+		get_pages(sbi, F2FS_DIRTY_META) < nr_pages_to_skip(sbi, META))
 		goto skip_write;
 
 	/* if mounting is failed, skip writing node pages */
 	mutex_lock(&sbi->cp_mutex);
-	written = sync_meta_pages(sbi, META, nrpages);
+	diff = nr_pages_to_write(sbi, META, wbc);
+	written = sync_meta_pages(sbi, META, wbc->nr_to_write);
 	mutex_unlock(&sbi->cp_mutex);
-	wbc->nr_to_write -= written;
+	wbc->nr_to_write = max((long)0, wbc->nr_to_write - written - diff);
 	return 0;
 
 skip_write:

fs/f2fs/data.c

@@ -844,8 +844,6 @@ static int f2fs_write_data_page(struct page *page,
 	return AOP_WRITEPAGE_ACTIVATE;
 }
 
-#define MAX_DESIRED_PAGES_WP	4096
-
 static int __f2fs_writepage(struct page *page, struct writeback_control *wbc,
 			void *data)
 {
@@ -862,7 +860,7 @@ static int f2fs_write_data_pages(struct address_space *mapping,
 	struct f2fs_sb_info *sbi = F2FS_SB(inode->i_sb);
 	bool locked = false;
 	int ret;
-	long excess_nrtw = 0, desired_nrtw;
+	long diff;
 
 	/* deal with chardevs and other special file */
 	if (!mapping->a_ops->writepage)
@@ -872,11 +870,7 @@ static int f2fs_write_data_pages(struct address_space *mapping,
 			get_dirty_dents(inode) < nr_pages_to_skip(sbi, DATA))
 		goto skip_write;
 
-	if (wbc->nr_to_write < MAX_DESIRED_PAGES_WP) {
-		desired_nrtw = MAX_DESIRED_PAGES_WP;
-		excess_nrtw = desired_nrtw - wbc->nr_to_write;
-		wbc->nr_to_write = desired_nrtw;
-	}
+	diff = nr_pages_to_write(sbi, DATA, wbc);
 
 	if (!S_ISDIR(inode->i_mode)) {
 		mutex_lock(&sbi->writepages);
@@ -890,7 +884,7 @@ static int f2fs_write_data_pages(struct address_space *mapping,
 
 	remove_dirty_dir_inode(inode);
 
-	wbc->nr_to_write -= excess_nrtw;
+	wbc->nr_to_write = max((long)0, wbc->nr_to_write - diff);
 	return ret;
 
 skip_write:

fs/f2fs/node.c

@@ -1202,7 +1202,7 @@ static int f2fs_write_node_pages(struct address_space *mapping,
 			    struct writeback_control *wbc)
 {
 	struct f2fs_sb_info *sbi = F2FS_SB(mapping->host->i_sb);
-	long nr_to_write = wbc->nr_to_write;
+	long diff;
 
 	/* balancing f2fs's metadata in background */
 	f2fs_balance_fs_bg(sbi);
@@ -1211,12 +1211,10 @@ static int f2fs_write_node_pages(struct address_space *mapping,
 	if (get_pages(sbi, F2FS_DIRTY_NODES) < nr_pages_to_skip(sbi, NODE))
 		goto skip_write;
 
-	/* if mounting is failed, skip writing node pages */
-	wbc->nr_to_write = 3 * max_hw_blocks(sbi);
+	diff = nr_pages_to_write(sbi, NODE, wbc);
 	wbc->sync_mode = WB_SYNC_NONE;
 	sync_node_pages(sbi, 0, wbc);
-	wbc->nr_to_write = nr_to_write - (3 * max_hw_blocks(sbi) -
-						wbc->nr_to_write);
+	wbc->nr_to_write = max((long)0, wbc->nr_to_write - diff);
 	return 0;
 
 skip_write:

fs/f2fs/segment.h

@@ -683,3 +683,27 @@ static inline int nr_pages_to_skip(struct f2fs_sb_info *sbi, int type)
 	else
 		return 0;
 }
+
+/*
+ * When writing pages, it'd better align nr_to_write for segment size.
+ */
+static inline long nr_pages_to_write(struct f2fs_sb_info *sbi, int type,
+					struct writeback_control *wbc)
+{
+	long nr_to_write, desired;
+
+	if (wbc->sync_mode != WB_SYNC_NONE)
+		return 0;
+
+	nr_to_write = wbc->nr_to_write;
+
+	if (type == DATA)
+		desired = 4096;
+	else if (type == NODE)
+		desired = 3 * max_hw_blocks(sbi);
+	else
+		desired = MAX_BIO_BLOCKS(max_hw_blocks(sbi));
+
+	wbc->nr_to_write = desired;
+	return desired - nr_to_write;
+}