netfs: Miscellaneous fixes

Fix some miscellaneous things in the new netfs lib[1]:

 (1) The kerneldoc for netfs_readpage() shouldn't say netfs_page().

 (2) netfs_readpage() can get an integer overflow on 32-bit when it
     multiplies page_index(page) by PAGE_SIZE.  It should use
     page_file_offset() instead.

 (3) netfs_write_begin() should use page_offset() to avoid the same
     overflow.

Note that netfs_readpage() needs to use page_file_offset() rather than
page_offset() as it may see swap-over-NFS.
Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/161789062190.6155.12711584466338493050.stgit@warthog.procyon.org.uk/ [1]

fs/netfs/read_helper.c

@@ -933,7 +933,7 @@ void netfs_readahead(struct readahead_control *ractl,
 EXPORT_SYMBOL(netfs_readahead);
 
 /**
- * netfs_page - Helper to manage a readpage request
+ * netfs_readpage - Helper to manage a readpage request
  * @file: The file to read from
  * @page: The page to read
  * @ops: The network filesystem's operations for the helper to use
@@ -968,7 +968,7 @@ int netfs_readpage(struct file *file,
 		return -ENOMEM;
 	}
 	rreq->mapping	= page_file_mapping(page);
-	rreq->start	= page_index(page) * PAGE_SIZE;
+	rreq->start	= page_file_offset(page);
 	rreq->len	= thp_size(page);
 
 	if (ops->begin_cache_operation) {
@@ -1106,7 +1106,7 @@ int netfs_write_begin(struct file *file, struct address_space *mapping,
 	if (!rreq)
 		goto error;
 	rreq->mapping		= page->mapping;
-	rreq->start		= page->index * PAGE_SIZE;
+	rreq->start		= page_offset(page);
 	rreq->len		= thp_size(page);
 	rreq->no_unlock_page	= page->index;
 	__set_bit(NETFS_RREQ_NO_UNLOCK_PAGE, &rreq->flags);