shmem mappings already contain exceptional entries where swap slot
information is remembered.

To be able to store eviction information for regular page cache,
prepare every site dealing with the radix trees directly to handle
entries other than pages.

The common lookup functions will filter out non-page entries and
return NULL for page cache holes, just as before.  But provide a raw
version of the API which returns non-page entries as well, and switch
shmem over to use it.


...

-/**
- * find_get_page - find and get a page reference
- * @mapping: the address_space to search
- * @offset: the page index
- *
- * Is there a pagecache struct page at the given (mapping, offset) tuple?
- * If yes, increment its refcount and return it; if no, return NULL.
- */
-struct page *find_get_page(struct address_space *mapping, pgoff_t offset)
+struct page *__find_get_page(struct address_space *mapping, pgoff_t offset)
 {
 	void **pagep;
 	struct page *page;

@@ -812,24 +828,31 @@ out:
 
 	return page;
 }
-EXPORT_SYMBOL(find_get_page);
+EXPORT_SYMBOL(__find_get_page);