Re: [PATCH] ARM: mm: reduce maximum number of CPUs if DEBUG_KMAP_LOCAL is enabled
From: Peter Robinson <hidden>
Date: 2021-02-16 23:08:49
On Tue, Feb 16, 2021 at 6:15 PM Ard Biesheuvel [off-list ref] wrote:
The debugging code for kmap_local() doubles the number of fixmap slots allocated for kmap_local(), in order to use half of them as guard regions. This causes the fixmap region to grow downwards and collide with the newly added virtual DT mapping, which is obviously not good. One manifestation of this is EFI boot on a kernel built with NR_CPUS=32, which may pass the FDT in highmem, resulting in block entries that the fixmap code misidentifies as table entries, and subsequently tries to dereference using a phys-to-virt translation that is only valid for lowmem. This results in a cryptic splat such as the one below. ftrace: allocating 45548 entries in 89 pages 8<--- cut here --- Unable to handle kernel paging request at virtual address fc6006f0 pgd = (ptrval) [fc6006f0] *pgd=80000040207003, *pmd=00000000 Internal error: Oops: a06 [#1] SMP ARM Modules linked in: CPU: 0 PID: 0 Comm: swapper Not tainted 5.11.0+ #382 Hardware name: Generic DT based system PC is at cpu_ca15_set_pte_ext+0x24/0x30 LR is at __set_fixmap+0xe4/0x118 pc : [<c041ac9c>] lr : [<c04189d8>] psr: 400000d3 sp : c1601ed8 ip : 00400000 fp : 00800000 r10: 0000071f r9 : 00421000 r8 : 00c00000 r7 : 00c00000 r6 : 0000071f r5 : ffade000 r4 : 4040171f r3 : 00c00000 r2 : 4040171f r1 : c041ac78 r0 : fc6006f0 Flags: nZcv IRQs off FIQs off Mode SVC_32 ISA ARM Segment none Control: 30c5387d Table: 40203000 DAC: 00000001 Process swapper (pid: 0, stack limit = 0x(ptrval)) So let's limit the supported number of CPUs to 16 when CONFIG_DEBUG_KMAP_LOCAL is in effect. Reported-by: Peter Robinson <redacted> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Peter Robinson <redacted> Tested with both a LPAE and non LPAE kernel.
quoted hunk ↗ jump to hunk
--- arch/arm/Kconfig | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig index 138248999df7..3d2c684eab77 100644 --- a/arch/arm/Kconfig +++ b/arch/arm/Kconfig@@ -1310,9 +1310,15 @@ config KASAN_SHADOW_OFFSET config NR_CPUS int "Maximum number of CPUs (2-32)" - range 2 32 + range 2 16 if DEBUG_KMAP_LOCAL + range 2 32 if !DEBUG_KMAP_LOCAL depends on SMP default "4" + help + The maximum number of CPUs that the kernel can support. + Up to 32 CPUs can be supported, or up to 16 if kmap_local() + debugging is enabled, which uses half of the per-CPU fixmap + slots as guard regions. config HOTPLUG_CPU bool "Support for hot-pluggable CPUs" --2.30.0
_______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel