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 <viro@zeniv.linux.org.uk>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1642,13 +1642,12 @@ repeat:
 	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;
 
@@ -1676,11 +1675,6 @@ repeat:
 			goto fail;
 		}
 	}
-out_update:
-	if (!PageUptodate(page))
-		SetPageUptodate(page);
-out_clear:
-	clear_cold_data(page);
 	return 0;
 
 fail:
@@ -1698,11 +1692,26 @@ static int f2fs_write_end(struct file *file,
 
 	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;