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

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Message-ID: <d5055b48-d722-e03d-fc32-16fd76e3fa22@csgroup.eu>
Date:   Mon, 9 May 2022 05:46:03 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        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



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.

Christophe