From e972a1a2b8098cd686db8b8e345af8c040401f0e Mon Sep 17 00:00:00 2001
From: Douglas Katzman <dougk@google.com>
Date: Tue, 7 Apr 2026 23:09:56 -0400
Subject: [PATCH] Halve the size of immobile cards

---
 src/compiler/x86-64/parms.lisp | 2 +-
 tests/checkheap.test.sh        | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/compiler/x86-64/parms.lisp b/src/compiler/x86-64/parms.lisp
index 8599d6bbb..463d8e002 100644
--- a/src/compiler/x86-64/parms.lisp
+++ b/src/compiler/x86-64/parms.lisp
@@ -42,7 +42,7 @@
 ;;; the start of objects.
 (defconstant gencgc-alloc-granularity 0)
 ;;; The card size for immobile/low space
-(defconstant immobile-card-bytes 4096)
+(defconstant immobile-card-bytes 2048)
 
 ;;; ### Note: 'lispword' always means 8 bytes, and 'word' usually means
 ;;; the same as 'lispword', except in the assembler and disassembler,
diff --git a/tests/checkheap.test.sh b/tests/checkheap.test.sh
index e010dbb23..ac130f211 100644
--- a/tests/checkheap.test.sh
+++ b/tests/checkheap.test.sh
@@ -13,8 +13,8 @@ then
   stdout=$TEST_DIRECTORY/$TEST_FILESTEM.out
   stderr=$TEST_DIRECTORY/$TEST_FILESTEM.err
   run_sbcl >$stdout 2>$stderr <<EOF
-;; Immobile pages have physical protection, so it's unlikely that it would be wrong.
-;; However, the verifier was never actually checking.
+;; Mark a page as logically protected when it should not be, and confirm that
+;; the heap invariant checker finds an error.
 (assert (= (sb-kernel:generation-of '*posix-argv*) sb-vm:+pseudo-static-generation+))
 (assert (< (sb-kernel:generation-of *posix-argv*) sb-vm:+pseudo-static-generation+))
 (define-alien-type nil
@@ -29,7 +29,7 @@ then
 (setf (extern-alien "pre_verify_gen_0" char) 1)
 (let ((i (floor (- (sb-kernel:get-lisp-obj-address '*posix-argv*)
                    sb-vm:fixedobj-space-start)
-                4096)))
+                sb-vm:immobile-card-bytes)))
   ;; Mark it as-if write-protected (though without using mprotect)
   ;; This constant is defined in immobile-space.c: #define WRITE_PROTECT 0x80
   (setf (slot (deref fixedobj-pages i) 'flags) 128)