f2fs: fix not to write data pages on the page reclaiming path

Even if f2fs_write_data_page is called by the page reclaiming path, we should
not write the page to provide enough free segments for the worst case scenario.
Otherwise, f2fs can face with no free segment while gc is conducted, resulting
in:

 ------------[ cut here ]------------
 kernel BUG at /home/zeus/f2fs_test/src/fs/f2fs/segment.c:565!
 RIP: 0010:[<ffffffffa02c3b11>]  [<ffffffffa02c3b11>] new_curseg+0x331/0x340 [f2fs]
 Call Trace:
  allocate_segment_by_default+0x204/0x280 [f2fs]
  allocate_data_block+0x108/0x210 [f2fs]
  write_data_page+0x8a/0xc0 [f2fs]
  do_write_data_page+0xe1/0x2a0 [f2fs]
  move_data_page+0x8a/0xf0 [f2fs]
  f2fs_gc+0x446/0x970 [f2fs]
  f2fs_balance_fs+0xb6/0xd0 [f2fs]
  f2fs_write_begin+0x50/0x350 [f2fs]
  ? unlock_page+0x27/0x30
  ? unlock_page+0x27/0x30
  generic_file_buffered_write+0x10a/0x280
  ? file_update_time+0xa3/0xf0
  __generic_file_aio_write+0x1c8/0x3d0
  ? generic_file_aio_write+0x52/0xb0
  ? generic_file_aio_write+0x52/0xb0
  generic_file_aio_write+0x65/0xb0
  do_sync_write+0x5a/0x90
  vfs_write+0xc5/0x1f0
  SyS_write+0x55/0xa0
  system_call_fastpath+0x16/0x1b
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/data.c

@@ -805,38 +805,30 @@ static int f2fs_write_data_page(struct page *page,
 
 	zero_user_segment(page, offset, PAGE_CACHE_SIZE);
 write:
-	if (unlikely(sbi->por_doing)) {
-		err = AOP_WRITEPAGE_ACTIVATE;
+	if (unlikely(sbi->por_doing))
 		goto redirty_out;
-	}
 
 	/* Dentry blocks are controlled by checkpoint */
 	if (S_ISDIR(inode->i_mode)) {
 		inode_dec_dirty_dents(inode);
 		err = do_write_data_page(page, &fio);
-	} else {
-		f2fs_lock_op(sbi);
-
-		if (f2fs_has_inline_data(inode) || f2fs_may_inline(inode)) {
-			err = f2fs_write_inline_data(inode, page, offset);
-			f2fs_unlock_op(sbi);
-			goto out;
-		} else {
-			err = do_write_data_page(page, &fio);
-		}
+		goto done;
+	}
 
-		f2fs_unlock_op(sbi);
+	if (!wbc->for_reclaim)
 		need_balance_fs = true;
-	}
-	if (err == -ENOENT)
-		goto out;
-	else if (err)
+	else if (has_not_enough_free_secs(sbi, 0))
 		goto redirty_out;
 
-	if (wbc->for_reclaim) {
-		f2fs_submit_merged_bio(sbi, DATA, WRITE);
-		need_balance_fs = false;
-	}
+	f2fs_lock_op(sbi);
+	if (f2fs_has_inline_data(inode) || f2fs_may_inline(inode))
+		err = f2fs_write_inline_data(inode, page, offset);
+	else
+		err = do_write_data_page(page, &fio);
+	f2fs_unlock_op(sbi);
+done:
+	if (err && err != -ENOENT)
+		goto redirty_out;
 
 	clear_cold_data(page);
 out:
@@ -848,7 +840,7 @@ static int f2fs_write_data_page(struct page *page,
 redirty_out:
 	wbc->pages_skipped++;
 	set_page_dirty(page);
-	return err;
+	return AOP_WRITEPAGE_ACTIVATE;
 }
 
 #define MAX_DESIRED_PAGES_WP	4096