[RFC 3/7] mm: add Kconfig options for page consistency checker

[Sasha Levin <sashal@kernel.org>]

From: Sasha Levin <sashal@nvidia.com>

Add two configuration options for the dual-bitmap page consistency
checker.

DEBUG_PAGE_CONSISTENCY enables the feature itself. It depends on
DEBUG_KERNEL since this is a debugging tool, and selects DEBUG_FS to
provide the statistics interface. Memory overhead is two bits per
physical page frame across two bitmaps, so about 1 MB for a 16 GB
system. The bitmaps are statically sized at boot from memblock, so
memory hotplug is not supported and the option depends on
!MEMORY_HOTPLUG.

DEBUG_PAGE_CONSISTENCY_PANIC controls the response to a detected
violation. When enabled (the default) the kernel panics on
double-alloc, double-free, or bitmap corruption; when disabled it
logs a warning and continues.

Based-on-patch-by: Sanif Veeras <sveeras@nvidia.com>
Assisted-by: Claude:claude-opus-4-7 <noreply@anthropic.com>
Signed-off-by: Sasha Levin <sashal@nvidia.com>
---
 mm/Kconfig.debug | 59 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index 7638d75b27db..a005c904677c 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -144,6 +144,65 @@ config PAGE_TABLE_CHECK_ENFORCED
 
 	  If unsure say "n".
 
+config DEBUG_PAGE_CONSISTENCY
+	bool "Debug page allocator with dual-bitmap consistency checking"
+	depends on DEBUG_KERNEL
+	depends on !MEMORY_HOTPLUG
+	select DEBUG_FS
+	help
+	  Enable dual-bitmap tracking of page allocations for corruption
+	  detection. Uses two complementary bitmaps where the invariant
+	  (primary == ~secondary) must hold. Any bit flip in either bitmap
+	  will be detected.
+
+	  This is useful for safety-critical systems requiring Freedom From
+	  Interference (FFI) guarantees per ISO 26262 (ASIL-D) and IEC 61508
+	  (SIL-3).
+
+	  When disabled, the hooks compile away. When enabled, a static key
+	  gates tracking until initialization succeeds. The bitmaps are flat,
+	  covering the entire PFN range from memblock_start_of_DRAM() to
+	  memblock_end_of_DRAM() including any holes. This is deliberate:
+	  simple (pfn - min_pfn) indexing is trivially auditable and avoids
+	  auxiliary data structures that could themselves be subject to
+	  corruption. Memory overhead is two bits per PFN in the spanned
+	  range, e.g. ~4 MB total for a 64 GB system. Waste from holes is
+	  typically under 2%.
+
+	  Based on NVIDIA safety research.
+
+	  If unsure, say N.
+
+config DEBUG_PAGE_CONSISTENCY_PANIC
+	bool "Panic on page consistency failure"
+	depends on DEBUG_PAGE_CONSISTENCY
+	default y
+	help
+	  If enabled, the kernel will panic when a page consistency
+	  violation is detected, such as double-alloc or double-free.
+
+	  If disabled, a WARN with a stack trace is emitted and execution
+	  continues.
+
+	  For safety-critical systems, say Y.
+	  For debugging/development, say N.
+
+config DEBUG_PAGE_CONSISTENCY_KUNIT_TEST
+	tristate "KUnit tests for dual-bitmap consistency primitives" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  Enable KUnit tests for the dual-bitmap primitives defined in
+	  <linux/dual_bitmap.h>. These tests verify the core algorithm:
+	  setting and clearing bits in complementary bitmaps, detecting
+	  double-set and double-clear conditions, and detecting simulated
+	  corruption.
+
+	  The tests exercise only the header-only dual_bitmap library and
+	  do not require CONFIG_DEBUG_PAGE_CONSISTENCY.
+
+	  If unsure, say N.
+
 config PAGE_POISONING
 	bool "Poison pages after freeing"
 	help
-- 
2.53.0