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/20241023171236.1122535-1-ziy@nvidia.com
Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Usama Arif <usamaarif642@gmail.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/migrate.c b/mm/migrate.c
index df91248755e49acd5d4e71c703c4be65d019d793..7e520562d421a83d7a1246aa8b1071414d4b716a 100644 (file)
--- a/mm/migrate.c
+++ b/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);