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Subject: Re: [PATCH] MIPS: fix pmd_mkinvalid
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc:     linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1657181495-33004-1-git-send-email-zhanghongchen@loongson.cn>
 <20220707092206.GA9894@alpha.franken.de>
 <bfb97f6b-7a39-8253-bc14-08f2f54ed312@loongson.cn>
 <0fd4e823-493f-07f0-08d5-f4a22491b602@loongson.cn>
 <20220712111917.GA10936@alpha.franken.de>
From:   Hongchen Zhang <zhanghongchen@loongson.cn>
Message-ID: <b2e74ba2-b831-d300-92f6-c410673e72cc@loongson.cn>
Date:   Tue, 12 Jul 2022 20:08:43 +0800
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On 2022/7/12 下午7:19, Thomas Bogendoerfer wrote:
> On Tue, Jul 12, 2022 at 06:01:08PM +0800, Hongchen Zhang wrote:
>> On 2022/7/7 下午7:12, Hongchen Zhang wrote:
>>> On 2022/7/7 下午5:22, Thomas Bogendoerfer wrote:
>>>> On Thu, Jul 07, 2022 at 04:11:35PM +0800, Hongchen Zhang wrote:
>>>>> When a pmd entry is invalidated by pmd_mkinvalid,pmd_present should
>>>>> return true.
>>>>> So introduce a _PMD_PRESENT_INVALID_SHIFT bit to check if a pmd is
>>>>> present but invalidated by pmd_mkinvalid.
>>>>
>>>> What problem are you trying to fix ? What are the symptoms ?
>>>>
>>>>> Reported-by: kernel test robot <lkp@intel.com>
>>>>
>>>> the test robot showed problems with your last version of the patch,
>>>> which hasn't been integrated into at least the MIPS tree, so no
>>>> need to that.
>>>>
>>>> Thomas.
>>>>
>>>
>>> Hi Thomas,
>>>     The idea come from the commit:
>>>     b65399f6111b(arm64/mm: Change THP helpers to comply with generic MM
>>>   semantics).
>>>     There is an problem now:
>>>           CPU 0        CPU 1
>>>       pmdp_invalidate        do_page_fault
>>>       ...              __handle_mm_fault
>>>                       is_swap_pmd == true
>>>                       trigger VM_BUG_ON() ?
>>>       set_pmd_at
>>>     the reason is that pmd_present return true,after this commit
>>>     pmd_present will return false,and the VM_BUG_ON will not be triggered.
>>>     Like arm64 does,we can introduce a new bit to fix this.
>>>
>>> Thanks.
>> Hi Thomas,
>>   Is there problem of this patch? What's your opinion of this patch?
> 
> I haven't dig deeper into it, but needing more page bits is a pain
> for 32bit kernel and would make it nearly impossible to get huge
> page support there. And the description you gave me, needs to be
> in the commit description.
> 
> Thomas.
> 
Hi Thomas,
  Thanks for your patiently review. For your question,
   1. I think there may be problem when compile 32bit kernel with huge 
page support,because _PAGE_HUGE_SHIFT is only defined for R4K now.
   2. I will modify the commit as you said and make a v2 patch.

Thanks.
Hongchen Zhang