mm: avoid VM_BUG_ON when try to map an anon large folio to zero page.

An anonymous large folio can be split into non order-0 folios,
try_to_map_unused_to_zeropage() should not VM_BUG_ON compound pages but
just return false.  This fixes the crash when splitting anonymous large
folios to non order-0 folios.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: b1f2020 ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Signed-off-by: Zi Yan <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Acked-by: Usama Arif <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Domenico Cerasuolo <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mike Rapoport (Microsoft) <[email protected]>
Cc: Nico Pache <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: x-y-z

mm/migrate.c

@@ -206,7 +206,8 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
 	pte_t newpte;
 	void *addr;
 
-	VM_BUG_ON_PAGE(PageCompound(page), page);
+	if (PageCompound(page))
+		return false;
 	VM_BUG_ON_PAGE(!PageAnon(page), page);
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);