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Message-ID: <87zfbikd4f.fsf@gmail.com>
Date: Fri, 29 Aug 2025 09:12:08 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Anshuman Khandual <anshuman.khandual@....com>, linux-arm-kernel@...ts.infradead.org
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Mark Brown <broonie@...nel.org>, Ryan Roberts <ryan.roberts@....com>, Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 3/3] arm64/ptdump: Add ARM64_PTDUMP_CONSOLE
Anshuman Khandual <anshuman.khandual@....com> writes:
> On 26/08/25 9:16 PM, Ritesh Harjani (IBM) wrote:
>> Anshuman Khandual <anshuman.khandual@....com> writes:
>>
>>> Enable early kernel page table dump for debug purpose when required via new
>>> config ARM64_PDUMP_CONSOLE. This calls existing ptdump_walk() early on just
>>> after ptdump has been initialized with ptdump_init().
>>
>> I happen to stumble upon this while looking for something else related
>> to ptdump and was curious to understand where this will be really
>> useful?
>>
>> So instead of dumping it via cat /sys/kernel/debug/kernel_page_tables,
>> this will dump at early boot during arch setup and before start_kernel().
>
> Right, primarily before vmalloc() space gets crowded. Also this provides
> an opportunity to do a diff between early boot and after boot kernel page
> table states.
Just want to understand this better - the diff here will mainly show the
new page table entries for the remaining layout which will get populated
in start_kernel() right? The existing mappings created during
setup_arch() won't get changed right?
>>
>> I was curious, since this anyway gets enabled only in debug kernels.
>> There we can always just boot with minimal busybox image which can jump
>> to shell quickly and dump the kernel page tables, correct?
>
> Here the kernel page table dump could happen earlier than that as well.
>>
>> Also is ARM64_PTDUMP_CONSOLE config option added on purpose? A kernel cmdline
>> like early_ptdump=yes|1|true could come much handy, right?
>
> Currently this is just for arm64 platform but could be enabled in general
> for other platforms as well. Yes, early_ptdump=yes|1|true will be useful
> as well. ARM64_PTDUMP_CONSOLE just build guards the additional code. But
> if required cmdline option "early_ptdump=" could just provide the runtime
> switch and then this could always be built enabled on CONFIG_PTDUMP.
>>
>> (Since I am fixing few issues on powerpc ptdump - hence was just curious
>> to know whether this can come useful for me too or not :) )
>
> This feature could be extended in general to other platforms. Let me know
> if you find this useful for powerpc.
Sure - as of now I don't have a direct usecase. But let me try to do
an early page table dump by taking some inspiration from your patch to
see if this adds value for powerpc case or not.
Thanks for sharing the info.
-ritesh
>>
>> Thanks!
>> -ritesh
>>
>>>
>>> Suggested-by: Ryan Roberts <ryan.roberts@....com>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>> ---
>>> arch/arm64/Kconfig.debug | 12 ++++++++++++
>>> arch/arm64/include/asm/ptdump.h | 7 +++++++
>>> arch/arm64/kernel/setup.c | 1 +
>>> arch/arm64/mm/ptdump.c | 7 +++++++
>>> 4 files changed, 27 insertions(+)
>>>
>>> diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
>>> index 265c4461031f..0f8af0dd0f4c 100644
>>> --- a/arch/arm64/Kconfig.debug
>>> +++ b/arch/arm64/Kconfig.debug
>>> @@ -20,4 +20,16 @@ config ARM64_RELOC_TEST
>>> depends on m
>>> tristate "Relocation testing module"
>>>
>>> +config ARM64_PTDUMP_CONSOLE
>>> + bool "Dump early kernel page table"
>>> + depends on DEBUG_KERNEL
>>> + depends on ARCH_HAS_PTDUMP
>>> + select PTDUMP
>>> + help
>>> + Enable this option to dump early kernel page table entries during
>>> + boot using the PTDUMP framework. This helps in examining kernel's
>>> + page table mapping entries and their attributes etc.
>>> +
>>> + If in doubt, say N.
>>> +
>>> source "drivers/hwtracing/coresight/Kconfig"
>>> diff --git a/arch/arm64/include/asm/ptdump.h b/arch/arm64/include/asm/ptdump.h
>>> index 27e774134e7f..81dc53ca9643 100644
>>> --- a/arch/arm64/include/asm/ptdump.h
>>> +++ b/arch/arm64/include/asm/ptdump.h
>>> @@ -74,8 +74,15 @@ void __init ptdump_debugfs_register(struct ptdump_info *info, const char *name);
>>> static inline void ptdump_debugfs_register(struct ptdump_info *info,
>>> const char *name) { }
>>> #endif /* CONFIG_PTDUMP_DEBUGFS */
>>> +
>>> +#ifdef CONFIG_ARM64_PTDUMP_CONSOLE
>>> +void __init arm64_kernel_pgtable_dump(void);
>>> +#else
>>> +static inline void __init arm64_kernel_pgtable_dump(void) { }
>>> +#endif /* CONFIG_ARM64_PTDUMP_CONSOLE */
>>> #else
>>> static inline void __init ptdump_init(void) { }
>>> +static inline void __init arm64_kernel_pgtable_dump(void) { }
>>> static inline void note_page(struct ptdump_state *pt_st, unsigned long addr,
>>> int level, pteval_t val) { }
>>> static inline void note_page_pte(struct ptdump_state *st, unsigned long addr, pte_t pte) { }
>>> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
>>> index 0a3812c8e177..86bf7607d304 100644
>>> --- a/arch/arm64/kernel/setup.c
>>> +++ b/arch/arm64/kernel/setup.c
>>> @@ -361,6 +361,7 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
>>> init_bootcpu_ops();
>>> smp_init_cpus();
>>> smp_build_mpidr_hash();
>>> + arm64_kernel_pgtable_dump();
>>>
>>> #ifdef CONFIG_ARM64_SW_TTBR0_PAN
>>> /*
>>> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
>>> index c78e6b496dea..f6d22462add6 100644
>>> --- a/arch/arm64/mm/ptdump.c
>>> +++ b/arch/arm64/mm/ptdump.c
>>> @@ -407,6 +407,13 @@ void __init ptdump_init(void)
>>> ptdump_initialize();
>>> }
>>>
>>> +#ifdef CONFIG_ARM64_PTDUMP_CONSOLE
>>> +void __init arm64_kernel_pgtable_dump(void)
>>> +{
>>> + ptdump_walk(CONSOLE, &kernel_ptdump_info);
>>> +}
>>> +#endif
>>> +
>>> static int __init ptdump_debugfs_init(void)
>>> {
>>> ptdump_debugfs_register(&kernel_ptdump_info, "kernel_page_tables");
>>> --
>>> 2.25.1
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