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In order to separate slabs from folios, we need to convert from any page in a slab to the slab directly without going through a page to folio conversion first. Up to this point, page_slab() has followed the example of other memdesc converters (page_folio(), page_ptdesc() etc) and just cast the pointer to the requested type, regardless of whether the pointer is actually a pointer to the correct type or not. That changes with this commit; we check that the page actually belongs to a slab and return NULL if it does not. Other memdesc converters will adopt this convention in future. kfence was the only user of page_slab(), so adjust it to the new way of working. It will need to be touched again when we separate slab from page. Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> Cc: Alexander Potapenko <glider@google.com> Cc: Marco Elver <elver@google.com> Cc: kasan-dev@googlegroups.com Link: https://patch.msgid.link/20251113000932.1589073-2-willy@infradead.org Acked-by: David Hildenbrand (Red Hat) <david@kernel.org> Tested-by: Marco Elver <elver@google.com> Reviewed-by: Harry Yoo <harry.yoo@oracle.com> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>