Re: [syzbot] [mm?] WARNING in page_add_anon_rmap

[Matthew Wilcox <willy@infradead.org>]

On Mon, Sep 18, 2023 at 12:00:24PM +0200, David Hildenbrand wrote:
> > If you want syzbot to run the reproducer, reply with:
> > #syz test: git://repo/address.git branch-or-commit-hash
> > If you attach or paste a git patch, syzbot will apply it before testing.

If I understand correctly, you have to do it like this:

#syz test

diff --git a/mm/rmap.c b/mm/rmap.c
index 789a2beb8b3a..f13a2927163d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1249,8 +1249,10 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 	}
 	if (flags & RMAP_EXCLUSIVE)
 		SetPageAnonExclusive(page);
-	VM_WARN_ON_FOLIO(page_mapcount(page) > 1 && PageAnonExclusive(page),
-			 folio);
+	/* While PTE-mapping a THP we have a PMD and a PTE mapping. */
+	VM_WARN_ON_FOLIO((atomic_read(&page->_mapcount) > 0 ||
+			  (folio_test_large(folio) && folio_entire_mapcount(folio) > 1)) &&
+			 PageAnonExclusive(page), folio);
 	mlock_vma_folio(folio, vma, compound);
 }
--