bcachefs: Fix short buffered writes

In the buffered write path, we have to check for short writes that write
to the full page, where the page wasn't UpToDate; when this happens, the
page is partly garbage, so we have to zero it out and revert that part
of the write.

This check was wrong - we reverted total from copied, but didn't revert
the iov_iter, probably also leading to corrupted writes.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io.c

@@ -1454,23 +1454,23 @@ static int __bch2_buffered_write(struct bch_inode_info *inode,
 		if (!pg_copied)
 			break;
 
+		if (!PageUptodate(page) &&
+		    pg_copied != PAGE_SIZE &&
+		    pos + copied + pg_copied < inode->v.i_size) {
+			zero_user(page, 0, PAGE_SIZE);
+			break;
+		}
+
 		flush_dcache_page(page);
 		copied += pg_copied;
+
+		if (pg_copied != pg_len)
+			break;
 	}
 
 	if (!copied)
 		goto out;
 
-	if (copied < len &&
-	    ((offset + copied) & (PAGE_SIZE - 1))) {
-		struct page *page = pages[(offset + copied) >> PAGE_SHIFT];
-
-		if (!PageUptodate(page)) {
-			zero_user(page, 0, PAGE_SIZE);
-			copied -= (offset + copied) & (PAGE_SIZE - 1);
-		}
-	}
-
 	spin_lock(&inode->v.i_lock);
 	if (pos + copied > inode->v.i_size)
 		i_size_write(&inode->v, pos + copied);
@@ -1567,6 +1567,7 @@ static ssize_t bch2_buffered_write(struct kiocb *iocb, struct iov_iter *iter)
 		}
 		pos += ret;
 		written += ret;
+		ret = 0;
 
 		balance_dirty_pages_ratelimited(mapping);
 	} while (iov_iter_count(iter));