The following commit has been merged into the x86/urgent branch of tip: Commit-ID: 3f0036c0b5f850d1200dbfa7365ed24197a0f157 Gitweb: https://git.kernel.org/tip/3f0036c0b5f850d1200dbfa7365ed24197a0f157 Author: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx> AuthorDate: Sun, 13 Apr 2025 11:08:58 +03:00 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitterDate: Sun, 13 Apr 2025 11:09:39 +02:00 x86/e820: Discard high memory that can't be addressed by 32-bit systems 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 ... Call Trace: memblock_free_pages+0x11/0x2c memblock_free_all+0x2ce/0x3a0 mm_core_init+0xf5/0x320 start_kernel+0x296/0x79c i386_start_kernel+0xad/0xb0 startup_32_smp+0x151/0x154 The mem_map[] is allocated up to the end of ZONE_HIGHMEM which is defined by max_pfn. The bug was introduced by this recent commit: 6faea3422e3b ("arch, mm: streamline HIGHMEM freeing") Previously, freeing of high memory was also clamped to the end of ZONE_HIGHMEM but after this change, 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. To fix this, discard the memory after max_pfn from memblock on 32-bit systems so that core MM would be aware only of actually usable memory. Fixes: 6faea3422e3b ("arch, mm: streamline HIGHMEM freeing") Reported-by: Dave Hansen <dave.hansen@xxxxxxxxx> Tested-by: Arnd Bergmann <arnd@xxxxxxxxxx> Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Andy Shevchenko <andy@xxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Davide Ciminaghi <ciminaghi@xxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: Sean Christopherson <seanjc@xxxxxxxxxx> Cc: kvm@xxxxxxxxxxxxxxx Link: https://lore.kernel.org/r/20250413080858.743221-1-rppt@xxxxxxxxxx --- arch/x86/kernel/e820.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c index 9d8dd8d..9920122 100644 --- a/arch/x86/kernel/e820.c +++ b/arch/x86/kernel/e820.c @@ -1299,6 +1299,14 @@ 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);
