linux-kernel - Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

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Message-ID: <f1f7f447-1b7b-9c42-f735-906d96b6077b@linux.alibaba.com>
Date:   Tue, 5 Jul 2022 10:44:43 +0800
From:   "guanghui.fgh" <guanghuifeng@...ux.alibaba.com>
To:     Will Deacon <will@...nel.org>
Cc:     baolin.wang@...ux.alibaba.com, catalin.marinas@....com,
        akpm@...ux-foundation.org, david@...hat.com, jianyong.wu@....com,
        james.morse@....com, quic_qiancai@...cinc.com,
        christophe.leroy@...roup.eu, jonathan@...ek.ca,
        mark.rutland@....com, thunder.leizhen@...wei.com,
        anshuman.khandual@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, rppt@...nel.org,
        geert+renesas@...der.be, ardb@...nel.org, linux-mm@...ck.org,
        yaohongbo@...ux.alibaba.com, alikernel-developer@...ux.alibaba.com
Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance
 degradation



在 2022/7/5 0:38, Will Deacon 写道:
> On Mon, Jul 04, 2022 at 10:34:07PM +0800, guanghui.fgh wrote:
>> Thanks.
>>
>> 在 2022/7/4 22:23, Will Deacon 写道:
>>> On Mon, Jul 04, 2022 at 10:11:27PM +0800, guanghui.fgh wrote:
>>>> 在 2022/7/4 21:15, Will Deacon 写道:
>>>>> On Mon, Jul 04, 2022 at 08:05:59PM +0800, guanghui.fgh wrote:
>>>>>>>> 1.Quoted messages from arch/arm64/mm/init.c
>>>>>>>>
>>>>>>>> "Memory reservation for crash kernel either done early or deferred
>>>>>>>> depending on DMA memory zones configs (ZONE_DMA) --
>>>>>>>>
>>>>>>>> In absence of ZONE_DMA configs arm64_dma_phys_limit initialized
>>>>>>>> here instead of max_zone_phys().  This lets early reservation of
>>>>>>>> crash kernel memory which has a dependency on arm64_dma_phys_limit.
>>>>>>>> Reserving memory early for crash kernel allows linear creation of block
>>>>>>>> mappings (greater than page-granularity) for all the memory bank rangs.
>>>>>>>> In this scheme a comparatively quicker boot is observed.
>>>>>>>>
>>>>>>>> If ZONE_DMA configs are defined, crash kernel memory reservation
>>>>>>>> is delayed until DMA zone memory range size initialization performed in
>>>>>>>> zone_sizes_init().  The defer is necessary to steer clear of DMA zone
>>>>>>>> memory range to avoid overlap allocation.
>>>>>>>>
>>>>>>>> [[[
>>>>>>>> So crash kernel memory boundaries are not known when mapping all bank memory
>>>>>>>> ranges, which otherwise means not possible to exclude crash kernel range
>>>>>>>> from creating block mappings so page-granularity mappings are created for
>>>>>>>> the entire memory range.
>>>>>>>> ]]]"
>>>>>>>>
>>>>>>>> Namely, the init order: memblock init--->linear mem mapping(4k mapping for
>>>>>>>> crashkernel, requirinig page-granularity changing))--->zone dma
>>>>>>>> limit--->reserve crashkernel.
>>>>>>>> So when enable ZONE DMA and using crashkernel, the mem mapping using 4k
>>>>>>>> mapping.
>>>>>>>
>>>>>>> Yes, I understand that is how things work today but I'm saying that we may
>>>>>>> as well leave the crashkernel mapped (at block granularity) if
>>>>>>> !can_set_direct_map() and then I think your patch becomes a lot simpler.
>>>>>>
>>>>>> But Page-granularity mapppings are necessary for crash kernel memory range
>>>>>> for shrinking its size via /sys/kernel/kexec_crash_size interfac(Quoted from
>>>>>> arch/arm64/mm/init.c).
>>>>>> So this patch split block/section mapping to 4k page-granularity mapping for
>>>>>> crashkernel mem.
>>>>>
>>>>> Why? I don't see why the mapping granularity is relevant at all if we
>>>>> always leave the whole thing mapped.
>>>>>
>>>> There is another reason.
>>>>
>>>> When loading crashkernel finish, the do_kexec_load will use
>>>> arch_kexec_protect_crashkres to invalid all the pagetable for crashkernel
>>>> mem(protect crashkernel mem from access).
>>>>
>>>> arch_kexec_protect_crashkres--->set_memory_valid--->...--->apply_to_pmd_range
>>>>
>>>> In the apply_to_pmd_range, there is a judement： BUG_ON(pud_huge(*pud)). And
>>>> if the crashkernel use block/section mapping, there will be some error.
>>>>
>>>> Namely, it's need to use non block/section mapping for crashkernel mem
>>>> before shringking.
>>>
>>> Well, yes, but we can change arch_kexec_[un]protect_crashkres() not to do
>>> that if we're leaving the thing mapped, no?
>>>
>> I think we should use arch_kexec_[un]protect_crashkres for crashkernel mem.
>>
>> Because when invalid crashkernel mem pagetable, there is no chance to rd/wr
>> the crashkernel mem by mistake.
>>
>> If we don't use arch_kexec_[un]protect_crashkres to invalid crashkernel mem
>> pagetable, there maybe some write operations to these mem by mistake which
>> may cause crashkernel boot error and vmcore saving error.
> 
> I don't really buy this line of reasoning. The entire main kernel is
> writable, so why do we care about protecting the crashkernel so much? The
> _code_ to launch the crash kernel is writable! If you care about preventing
> writes to memory which should not be writable, then you should use
> rodata=full.
> 
> Will
Thanks.

1.I think the normal kernel mem can be writeable or readonly

2.But the normal kernel should't access(read/write) crashkernel mem 
after the crashkernel is loaded to the mem.(despite the normal kernel 
write/read access protect)
So invalid pagetable for crashkernel mem is needed.

With this method, it can guarantee the usability of the crashkernel when 
occuring emergency and generating vmcore.