Re: [PATCH V2] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask

[Li Xinhai <lixinhai.lxh@gmail.com>]

On 12/11/21 11:16 PM, Li Xinhai wrote:
> 
> 
> On 12/11/21 11:10 PM, Li Xinhai wrote:
>> When BUG_ON check for THP migration entry, the exsiting code only check
>> thp_migration_supported case, but not for !thp_migration_supported case.
>> To make the BUG_ON check consistent, we need catch both cases.
>>
>> Move the BUG_ON check one step eariler, because if the bug happen we
>> should know it instead of depend on FOLL_MIGRATION been used by caller.
>>
>> Because pmdval instead of *pmd is read by the is_pmd_migration_entry()
>> check, the existing code don't help to avoid useless locking within
>> pmd_migration_entry_wait(), so remove that check.
>>
>> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
>> ---
> V1->V2:
> Move the BUG_ON() check before if(!(flags & FOLL_MIGRATION)); and add comments
> for it.
> 
Yan, Ying and Kirill have been worked on this part of code, may help to review.

This change was based on my code review, no real bug has been observed.


>>   mm/gup.c | 13 +++++++++----
>>   1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 2c51e9748a6a..94d0e586ca0b 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>>       }
>>   retry:
>>       if (!pmd_present(pmdval)) {
>> +        /*
>> +         * Should never reach here, if thp migration is not supported;
>> +         * Otherwise, it must be a thp miration entry.
>> +         */
>> +        VM_BUG_ON(!thp_migration_supported() ||
>> +                  !is_pmd_migration_entry(pmdval));
>> +
>>           if (likely(!(flags & FOLL_MIGRATION)))
>>               return no_page_table(vma, flags);
>> -        VM_BUG_ON(thp_migration_supported() &&
>> -                  !is_pmd_migration_entry(pmdval));
>> -        if (is_pmd_migration_entry(pmdval))
>> -            pmd_migration_entry_wait(mm, pmd);
>> +
>> +        pmd_migration_entry_wait(mm, pmd);
>>           pmdval = READ_ONCE(*pmd);
>>           /*
>>            * MADV_DONTNEED may convert the pmd to null because
>>