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

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Date:   Mon, 4 Jul 2022 22:11:27 +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 2022/7/4 21:15, Will Deacon 写道:
> On Mon, Jul 04, 2022 at 08:05:59PM +0800, guanghui.fgh wrote:
>>
>>
>> 在 2022/7/4 19:14, Will Deacon 写道:
>>> On Mon, Jul 04, 2022 at 06:58:20PM +0800, guanghui.fgh wrote:
>>>>
>>>>
>>>> 在 2022/7/4 18:35, Will Deacon 写道:
>>>>> On Sat, Jul 02, 2022 at 11:57:53PM +0800, Guanghui Feng wrote:
>>>>>> The arm64 can build 2M/1G block/sectiion mapping. When using DMA/DMA32 zone
>>>>>> (enable crashkernel, disable rodata full, disable kfence), the mem_map will
>>>>>> use non block/section mapping(for crashkernel requires to shrink the region
>>>>>> in page granularity). But it will degrade performance when doing larging
>>>>>> continuous mem access in kernel(memcpy/memmove, etc).
>>>>>
>>>>> Hmm. It seems a bit silly to me that we take special care to unmap the
>>>>> crashkernel from the linear map even when can_set_direct_map() is false, as
>>>>> we won't be protecting the main kernel at all!
>>>>>
>>>>> Why don't we just leave the crashkernel mapped if !can_set_direct_map()
>>>>> and then this problem just goes away?
>>>>
>>>> This question had been asked lask week.
>>>
>>> Sorry, I didn't spot that. Please could you link me to the conversation, as
>>> I'm still unable to find it in my inbox?
>>
>> Please access this link:
>> https://lore.kernel.org/linux-arm-kernel/075b0a8e-cb7e-70f6-b45a-54cd31886794@linux.alibaba.com/T/
> 
> Sorry, but I read through the thread and I still can't find where the
> possibility of leaving the crashkernel mapped was discussed.
> 
>>>> 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.
> 
> Will

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.

Thanks.