f2fs: fix to set PageUptodate in f2fs_write_end correctly

Previously, f2fs_write_begin sets PageUptodate all the time. But, when user
tries to update the entire page (i.e., len == PAGE_SIZE), we need to consider
that the page is able to be copied partially afterwards. In such the case,
we will lose the remaing region in the page.

This patch fixes this by setting PageUptodate in f2fs_write_end as given copied
result. In the short copy case, it returns zero to let generic_perform_write
retry copying user data again.

As a result, f2fs_write_end() works:
PageUptodate len copied return retry
1. no 4096 4096 4096 false -> return 4096
2. no 4096 1024 0 true -> goto #1 case
3. yes 2048 2048 2048 false -> return 2048
4. yes 2048 1024 1024 false -> return 1024

Suggested-by: Al Viro <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/data.c

···
 	if (f2fs_encrypted_inode(inode) && S_ISREG(inode->i_mode))
 		f2fs_wait_on_encrypted_page_writeback(sbi, blkaddr);
 
-	if (len == PAGE_SIZE)
-		goto out_update;
-	if (PageUptodate(page))
-		goto out_clear;
+	if (len == PAGE_SIZE || PageUptodate(page))
+		return 0;
 
 	if (blkaddr == NEW_ADDR) {
 		zero_user_segment(page, 0, PAGE_SIZE);
+		SetPageUptodate(page);
 	} else {
 		struct bio *bio;
 
···
 			goto fail;
 		}
 	}
-out_update:
-	if (!PageUptodate(page))
-		SetPageUptodate(page);
-out_clear:
-	clear_cold_data(page);
 	return 0;
 
 fail:
···
 
 	trace_f2fs_write_end(inode, pos, len, copied);
 
+	/*
+	 * This should be come from len == PAGE_SIZE, and we expect copied
+	 * should be PAGE_SIZE. Otherwise, we treat it with zero copied and
+	 * let generic_perform_write() try to copy data again through copied=0.
+	 */
+	if (!PageUptodate(page)) {
+		if (unlikely(copied != PAGE_SIZE))
+			copied = 0;
+		else
+			SetPageUptodate(page);
+	}
+	if (!copied)
+		goto unlock_out;
+
 	set_page_dirty(page);
+	clear_cold_data(page);
 
 	if (pos + copied > i_size_read(inode))
 		f2fs_i_size_write(inode, pos + copied);
-
+unlock_out:
 	f2fs_put_page(page, 1);
 	f2fs_update_time(F2FS_I_SB(inode), REQ_TIME);
 	return copied;