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>
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 4e0e7ba..7f4665e 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -851,8 +851,6 @@
 	return AOP_WRITEPAGE_ACTIVATE;
 }
 
-#define MAX_DESIRED_PAGES_WP	4096
-
 static int __f2fs_writepage(struct page *page, struct writeback_control *wbc,
 			void *data)
 {
@@ -869,7 +867,7 @@
 	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)
@@ -879,11 +877,7 @@
 			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);
@@ -897,7 +891,7 @@
 
 	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: