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

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Date:   Tue, 10 May 2022 09:35:40 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Muchun Song <songmuchun@...edance.com>
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        <James.Bottomley@...senPartnership.com>,
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Subject: Re: [PATCH v2 1/3] mm: change huge_ptep_clear_flush() to return the
 original pte



On 5/10/2022 4:02 AM, Mike Kravetz wrote:
> On 5/9/22 01:46, Baolin Wang wrote:
>>
>>
>> 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.
>>
> 
> Sorry for the confusion.
> 
> In the original commit, it seemed odd to me that the signature of the
> function was changing and there was not an associated change to the only
> caller of the function.  I did suggest casting to void or adding a comment.
> As Christophe mentions, the cast to void is not necessary.  In addition,
> there really isn't a need for a comment as the calling code is not changed.

OK. Will drop the casting in next version.

> 
> The original version of the commit without either is actually preferable.
> The commit message does say this is a preparation patch and the return
> value will be used in later patches.

OK. Thanks Mike for making me clear. Also thanks to Muchun and Christophe.