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

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Date:   Thu, 7 Jul 2022 19:12:49 +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.