mm/rmap: simplify PageAnonExclusive sanity checks when adding anon rmap

Let's sanity-check PageAnonExclusive vs.  mapcount in page_add_anon_rmap()
and hugepage_add_anon_rmap() after setting PageAnonExclusive simply by
re-reading the mapcounts.

We can stop initializing the "first" variable in page_add_anon_rmap() and
no longer need an atomic_inc_and_test() in hugepage_add_anon_rmap().

While at it, switch to VM_WARN_ON_FOLIO().

[david@redhat.com: update check for doubly-mapped page]
  Link: https://lkml.kernel.org/r/d8e5a093-2e22-c14b-7e64-6da280398d9f@redhat.com
Link: https://lkml.kernel.org/r/20230913125113.313322-6-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/rmap.c

@@ -1199,7 +1199,7 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 	atomic_t *mapped = &folio->_nr_pages_mapped;
 	int nr = 0, nr_pmdmapped = 0;
 	bool compound = flags & RMAP_COMPOUND;
-	bool first = true;
+	bool first;
 
 	/* Is page being mapped by PTE? Is this its first map to be added? */
 	if (likely(!compound)) {
@@ -1228,9 +1228,6 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 		}
 	}
 
-	VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
-	VM_BUG_ON_PAGE(!first && PageAnonExclusive(page), page);
-
 	if (nr_pmdmapped)
 		__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr_pmdmapped);
 	if (nr)
@@ -1252,6 +1249,10 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 	}
 	if (flags & RMAP_EXCLUSIVE)
 		SetPageAnonExclusive(page);
+	/* 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);
 }
@@ -2545,15 +2546,14 @@ void hugepage_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 			    unsigned long address, rmap_t flags)
 {
 	struct folio *folio = page_folio(page);
-	int first;
 
 	VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
 
-	first = atomic_inc_and_test(&folio->_entire_mapcount);
-	VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
-	VM_BUG_ON_PAGE(!first && PageAnonExclusive(page), page);
+	atomic_inc(&folio->_entire_mapcount);
 	if (flags & RMAP_EXCLUSIVE)
 		SetPageAnonExclusive(page);
+	VM_WARN_ON_FOLIO(folio_entire_mapcount(folio) > 1 &&
+			 PageAnonExclusive(page), folio);
 }
 
 void hugepage_add_new_anon_rmap(struct folio *folio,