linux-kernel - Re: [PATCH] mm/swapops: make is_pmd_migration

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Message-ID: <752b59ff-3283-2360-5f16-d75d04c25a8b@loongson.cn>
Date:   Fri, 29 Apr 2022 10:23:08 +0800
From:   Hongchen Zhang <zhanghongchen@...ngson.cn>
To:     Peter Xu <peterx@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/swapops: make is_pmd_migration_entry more strict

On 2022/4/29 上午2:41, Peter Xu wrote:
> On Thu, Apr 28, 2022 at 03:35:33PM +0800, Hongchen Zhang wrote:
>> a pmd migration entry should first be a swap pmd,so
>> use is_swap_pmd(pmd) instead of !pmd_present(pmd).
>>
>> On the other hand, some architecture (MIPS for example)
>> may misjudge a pmd_none entry as a pmd migration entry.
>>
>> Signed-off-by: Hongchen Zhang <zhanghongchen@...ngson.cn>
> 
> The change looks reasonable,
> 
> Acked-by: Peter Xu <peterx@...hat.com>
> 
> Two more pure questions..
> 
>    (1) is there a real issue to be fixed with it?  Asked because I see most
>        calls to is_pmd_migration_entry() should already make sure it's not
>        none, and,
> 
>    (2) why it matters with MIPS?  Firstly this function is not used in arch
>        specific code, and iiuc thp migration is not enabled at all for mips.
> 
> Thanks,
> 
>> ---
>>   include/linux/swapops.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
>> index d356ab4..1d16569 100644
>> --- a/include/linux/swapops.h
>> +++ b/include/linux/swapops.h
>> @@ -304,7 +304,7 @@ static inline pmd_t swp_entry_to_pmd(swp_entry_t entry)
>>   
>>   static inline int is_pmd_migration_entry(pmd_t pmd)
>>   {
>> -	return !pmd_present(pmd) && is_migration_entry(pmd_to_swp_entry(pmd));
>> +	return is_swap_pmd(pmd) && is_migration_entry(pmd_to_swp_entry(pmd));
>>   }
>>   #else
>>   static inline void set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
>> -- 
>> 1.8.3.1
>>
> 
Hello Peter,
   (1) the pmd passed to __split_huge_pmd_locked may be a none pmd,so it 
may be wrongly treated as a pmd migration entry.
   (2) we are preparing to support thp migration on MIPS.

Thanks.