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

["guanghui.fgh" <guanghuifeng@linux.alibaba.com>]

在 2022/7/5 20:11, Will Deacon 写道:
> On Tue, Jul 05, 2022 at 08:07:07PM +0800, guanghui.fgh wrote:
>>
>>
>> 在 2022/7/5 17:52, Will Deacon 写道:
>>> On Mon, Jul 04, 2022 at 07:09:23PM +0200, Ard Biesheuvel wrote:
>>>> On Mon, 4 Jul 2022 at 18:38, Will Deacon <will@kernel.org> wrote:
>>>>>
>>>>> 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:
>>>> ...
>>>>>>>> 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.
>>>>>
>>>>
>>>> This is not entirely true - the core kernel text and rodata are
>>>> remapped r/o in the linear map, whereas all module code and rodata are
>>>> left writable when rodata != full.
>>>
>>> Yes, sorry, you're quite right. The kernel text is only writable if
>>> rodata=off.
>>>
>>> But I still think it makes sense to protect the crashkernel only if
>>> rodata=full (which is the default on arm64) as this allows us to rely
>>> on page mappings and I think fits well with what we do for modules.
>>>
>>>> But the conclusion is the same, imo: if you can't be bothered to
>>>> protect a good chunk of the code and rodata that the kernel relies on,
>>>> why should the crashkernel be treated any differently?
>>>
>>> Thanks.
>>>
>>> Will
>> Thanks.
>>
>> 1.The rodata full is harm to the performance and has been disabled in-house.
>>
>> 2.When using crashkernel with rodata non full, the kernel also will use non
>> block/section mapping which cause high d-TLB miss and degrade performance
>> greatly.
>> This patch fix it to use block/section mapping as far as possible.
>>
>> bool can_set_direct_map(void)
>> {
>> 	return rodata_full || debug_pagealloc_enabled();
>> }
>>
>> map_mem:
>> if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
>> 	flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>>
>> 3.When rodata full is disabled, crashkernel also need protect(keep
>> arch_kexec_[un]protect_crashkres using).
>> I think crashkernel should't depend on radata full(Maybe other architecture
>> don't support radata full now).
> 
> I think this is going round in circles :/
> 
> As a first step, can we please leave the crashkernel mapped unless
> rodata=full? It should be a much simpler patch to write, review and maintain
> and it gives you the performance you want when crashkernel is being used.
> 
> Will

Thanks.

There is a circle.

1.When the rodata is non full, there will be some error when calling 
arch_kexec_[un]protect_crashkres(BUG_ON(pud_huge(*pud))) now.
It's also need non-block/section mapping for crashkernel mem.

2.In other words, maybe we should change 
arch_kexec_[un]protect_crashkres to support block/section mapping which 
can leave crashkernel block/section mapping.

But when we shrink the crashkernel mem, we also need to split some 
block/section mapping(part mem for crashkernel, the left for the normal 
kernel).
As a result, maybe we build crashkernel mem with non-block/section 
mapping is appropriate(as this patch doing).