linux-kernel - Re: [PATCH v2 1/3] mm: change huge_ptep_clear

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Date:   Mon, 9 May 2022 16:46:03 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Muchun Song <songmuchun@...edance.com>
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Subject: Re: [PATCH v2 1/3] mm: change huge_ptep_clear_flush() to return the
 original pte



On 5/9/2022 1:46 PM, Christophe Leroy wrote:
> 
> 
> Le 08/05/2022 à 15:09, Baolin Wang a écrit :
>>
>>
>> On 5/8/2022 7:09 PM, Muchun Song wrote:
>>> On Sun, May 08, 2022 at 05:36:39PM +0800, Baolin Wang wrote:
>>>> It is incorrect to use ptep_clear_flush() to nuke a hugetlb page
>>>> table when unmapping or migrating a hugetlb page, and will change
>>>> to use huge_ptep_clear_flush() instead in the following patches.
>>>>
>>>> So this is a preparation patch, which changes the
>>>> huge_ptep_clear_flush()
>>>> to return the original pte to help to nuke a hugetlb page table.
>>>>
>>>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>>> Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
>>>
>>> Reviewed-by: Muchun Song <songmuchun@...edance.com>
>>
>> Thanks for reviewing.
>>
>>>
>>> But one nit below:
>>>
>>> [...]
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index 8605d7e..61a21af 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -5342,7 +5342,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct
>>>> *mm, struct vm_area_struct *vma,
>>>>            ClearHPageRestoreReserve(new_page);
>>>>            /* Break COW or unshare */
>>>> -        huge_ptep_clear_flush(vma, haddr, ptep);
>>>> +        (void)huge_ptep_clear_flush(vma, haddr, ptep);
>>>
>>> Why add a "(void)" here? Is there any warning if no "(void)"?
>>> IIUC, I think we can remove this, right?
>>
>> I did not meet any warning without the casting, but this is per Mike's
>> comment[1] to make the code consistent with other functions casting to
>> void type explicitly in hugetlb.c file.
>>
>> [1]
>> https://lore.kernel.org/all/495c4ebe-a5b4-afb6-4cb0-956c1b18d0cc@oracle.com/
>>
> 
> As far as I understand, Mike said that you should be accompagnied with a
> big fat comment explaining why we ignore the returned value from
> huge_ptep_clear_flush(). >
> By the way huge_ptep_clear_flush() is not declared 'must_check' so this
> cast is just visual polution and should be removed.
> 
> In the meantime the comment suggested by Mike should be added instead.
Sorry for my misunderstanding. I just follow the explicit void casting 
like other places in hugetlb.c file. And I am not sure if it is useful 
adding some comments like below, since we did not need the original pte 
value in the COW case mapping with a new page, and the code is more 
readable already I think.

Mike, could you help to clarify what useful comments would you like? and 
remove the explicit void casting? Thanks.

/*
  * Just ignore the return value with new page mapped.
  */