linux-kernel - Re: [PATCH] MIPS: fix pmd

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Date:   Tue, 12 Jul 2022 18:01:08 +0800
From:   Hongchen Zhang <zhanghongchen@...ngson.cn>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MIPS: fix pmd_mkinvalid

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@...el.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?

Thanks