mm/rmap: cleanup partially-mapped handling in __folio_remove_rmap()

Let's simplify and reduce code indentation.  In the RMAP_LEVEL_PTE case,
we already check for nr when computing partially_mapped.

For RMAP_LEVEL_PMD, it's a bit more confusing.  Likely, we don't need the
"nr" check, but we could have "nr < nr_pmdmapped" also if we stumbled into
the "/* Raced ahead of another remove and an add?  */" case.  So let's
simply move the nr check in there.

Note that partially_mapped is always false for small folios.

No functional change intended.

Link: https://lkml.kernel.org/r/20240710214350.147864-1-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/rmap.c

@@ -1568,22 +1568,19 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 			}
 		}
 
-		partially_mapped = nr < nr_pmdmapped;
+		partially_mapped = nr && nr < nr_pmdmapped;
 		break;
 	}
 
-	if (nr) {
-		/*
-		 * Queue anon large folio for deferred split if at least one
-		 * page of the folio is unmapped and at least one page
-		 * is still mapped.
-		 *
-		 * Check partially_mapped first to ensure it is a large folio.
-		 */
-		if (folio_test_anon(folio) && partially_mapped &&
-		    list_empty(&folio->_deferred_list))
-			deferred_split_folio(folio);
-	}
+	/*
+	 * Queue anon large folio for deferred split if at least one page of
+	 * the folio is unmapped and at least one page is still mapped.
+	 *
+	 * Check partially_mapped first to ensure it is a large folio.
+	 */
+	if (partially_mapped && folio_test_anon(folio) &&
+	    list_empty(&folio->_deferred_list))
+		deferred_split_folio(folio);
 	__folio_mod_stat(folio, -nr, -nr_pmdmapped);
 
 	/*