x86/e820: discard high memory that can't be addressed by 32-bit systems

Commit Message

Mike Rapoport April 13, 2025, 8:08 a.m. UTC

From: "Mike Rapoport (Microsoft)" <[e-mail]>

Dave Hansen reports the following crash on a 32-bit system with
CONFIG_HIGHMEM=y and CONFIG_X86_PAE=y:

  > 0xf75fe000 is the mem_map[] entry for the first page >4GB. It
  > obviously wasn't allocated, thus the oops.

  BUG: unable to handle page fault for address: f75fe000
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  *pdpt = 0000000002da2001 *pde = 000000000300c067 *pte = 0000000000000000
  Oops: Oops: 0002 [#1] SMP NOPTI
  CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.15.0-rc1-00288-ge618ee89561b-dirty #311 PREEMPT(undef)
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  EIP: __free_pages_core+0x3c/0x74
  Code: c3 d3 e6 83 ec 10 89 44 24 08 89 74 24 04 c7 04 24 c6 32 3a c2 89 55 f4 e8 a9 11 45 fe 85 f6 8b 55 f4 74 19 89 d8 31 c9 66 90 <0f> ba 30 0d c7 40 1c 00 00 00 00 41 83 c0 28 39 ce 75 ed 8b

  EAX: f75fe000 EBX: f75fe000 ECX: 00000000 EDX: 0000000a
  ESI: 00000400 EDI: 00500000 EBP: c247becc ESP: c247beb4
  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068 EFLAGS: 00210046
  CR0: 80050033 CR2: f75fe000 CR3: 02da6000 CR4: 000000b0
  Call Trace:
   memblock_free_pages+0x11/0x2c
   memblock_free_all+0x2ce/0x3a0
   mm_core_init+0xf5/0x320
   start_kernel+0x296/0x79c
   ? set_init_arg+0x70/0x70
   ? load_ucode_bsp+0x13c/0x1a8
   i386_start_kernel+0xad/0xb0
   startup_32_smp+0x151/0x154
  Modules linked in:
  CR2: 00000000f75fe000

The mem_map[] is allocated up to the end of ZONE_HIGHMEM which is defined
by max_pfn.

Before 6faea3422e3b ("arch, mm: streamline HIGHMEM freeing") freeing of
high memory was also clamped to the end of ZONE_HIGHMEM but after
6faea3422e3b memblock_free_all() tries to free memory above the of
ZONE_HIGHMEM as well and that causes access to mem_map[] entries beyond
the end of the memory map.

Discard the memory after max_pfn from memblock on 32-bit systems so that
core MM would be aware only of actually usable memory.

Reported-by: Dave Hansen <[e-mail]>
Tested-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Mike Rapoport (Microsoft) <[e-mail]>
---
 arch/x86/kernel/e820.c | 8 ++++++++
 1 file changed, 8 insertions(+)


base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8

Patch

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 57120f0749cc..5f673bd6c7d7 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -1300,6 +1300,14 @@  void __init e820__memblock_setup(void)
 		memblock_add(entry->addr, entry->size);
 	}
 
+	/*
+	 * 32-bit systems are limited to 4BG of memory even with HIGHMEM and
+	 * to even less without it.
+	 * Discard memory after max_pfn - the actual limit detected at runtime.
+	 */
+	if (IS_ENABLED(CONFIG_X86_32))
+		memblock_remove(PFN_PHYS(max_pfn), -1);
+
 	/* Throw away partial pages: */
 	memblock_trim_memory(PAGE_SIZE);