Re: [PATCH V3] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask

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

On 12/17/21 11:01 AM, Huang, Ying wrote:
> Li Xinhai <lixinhai.lxh@gmail.com> writes:
> 
>> When BUG_ON check for THP migration entry, the exsiting code only check
> 
> s/exsiting/existing/
> 
> Found some misspelling in the comments too.  Please fix them with some
> tool.
> 
Yes, those need be fixed, thanks.

> Best Regards,
> Huang, Ying
> 
>> thp_migration_supported case, but not for !thp_migration_supported case.
>> If !thp_migration_supported() and !pmd_present(), the original code may
>> dead loop in theory. 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>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: "Huang, Ying" <ying.huang@intel.com>
>> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>> ---
>> V2->V3:
>> mention about the dead loop in commit message.
>>
>> V1->V2:
>> Move the BUG_ON() check before if(!(flags & FOLL_MIGRATION)); and add comments
>> for it.
>>
>>
>>   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