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

["Huang, Ying" <ying.huang@intel.com>]

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Li Xinhai <lixinhai.lxh@gmail.com> writes:

> On 12/15/21 10:46 PM, Zi Yan wrote:
>> On 15 Dec 2021, at 7:47, Li Xinhai wrote:
>> 
>>> 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));
>>>>> +
>> This means VM_BUG_ON will be triggered when pmdval is not present
>> and THP migration
>> is not enabled. This can happen when it is pmd_none(). That is not a bug and should
>> just return no_page_table(vma, flags).
>> 
>
> Thanks for review!
> The pmd_none() has been checked at the beginning of follow_pmd_mask() and before
> the 'retry' loop start again, so that possibility is excluded.
>
> We will have VM_BUG_ON(1) if thp_migration_supported() is false, or
> VM_BUG_ON(!is_pmd_migration_entry(pmdval)) if thp_migration_supported() is true, by
> compiler.
>
> If we left !thp_migration_supported() case for the VM_BUG_ON(), it will cause
> misunderstanding that that case is not a bug at the code context.

I think your code works.  The patch description may be improved.  If
!thp_migration_supported() and !pmd_present(), the original code may
dead loop in theory.

Best Regards,
Huang, Ying

>>>>>       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
>>>>>
>> --
>> Best Regards,
>> Yan, Zi
>>