Re: [PATCH] arm64/mm/hotplug: Warn when memory limit has been reduced
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From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2021-09-16 14:34:38
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On Tue, Sep 14, 2021 at 12:21:10PM +0530, Anshuman Khandual wrote:
If the max memory limit has been reduced with 'mem=' kernel command line option, there might be UEFI memory map described memory beyond that limit which could be hot removed. This might be problematic for subsequent kexec kernel which could just access such removed memory. Memory offline notifier exists because there is no other way to block the removal of boot memory, only the offlining (which isn't actually a problem) But with 'mem=', there is no chance to stop such boot memory being offlined as it where never in use by the kernel. As 'mem=' is a debug only option on arm64 platform, just warn for such a situation and move on.
Just to make sure I understand, is the memory beyond the mem= limit considered online by the core code and it can be subsequently offlined? Looking at walk_system_ram_range(), it doesn't seem to care about the removed memblock ranges. Would such memory beyond the mem= limit need to have been onlined first?
quoted hunk ↗ jump to hunk
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index cfd9deb347c3..7ac39ee876c3 100644 --- a/arch/arm64/mm/mmu.c +++ b/arch/arm64/mm/mmu.c@@ -1627,6 +1627,18 @@ static int __init prevent_bootmem_remove_init(void) if (!IS_ENABLED(CONFIG_MEMORY_HOTREMOVE)) return ret; + if (has_mem_limit_reduced()) { + /* + * Physical memory limit has been reduced via the 'mem=' kernel + * command line option. Memory beyond reduced limit could now be + * removed and reassigned (guest ?) transparently to the kernel. + * This might cause subsequent kexec kernel to crash or at least + * corrupt the memory when accessing UEFI memory map enumerated + * boot memory which might have been repurposed. + */ + pr_warn("Memory limit reduced, kexec might be problematic\n"); + }
I'd actually move the warning to hotplug notifier callback rather than the init function. I'd also make it pr_warn_once(). -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel