Re: [PATCH] MADVISE_FREE, THP: Fix madvise_free_huge_pmd return value after splitting

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

Minchan Kim <minchan@kernel.org> writes:

> Hi,
>
> On Thu, Jun 16, 2016 at 08:03:54PM -0700, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@intel.com>
>> 
>> madvise_free_huge_pmd should return 0 if the fallback PTE operations are
>> required.  In madvise_free_huge_pmd, if part pages of THP are discarded,
>> the THP will be split and fallback PTE operations should be used if
>> splitting succeeds.  But the original code will make fallback PTE
>> operations skipped, after splitting succeeds.  Fix that via make
>> madvise_free_huge_pmd return 0 after splitting successfully, so that the
>> fallback PTE operations will be done.
>
> You're right. Thanks!
>
>> 
>> Know issues: if my understanding were correct, return 1 from
>> madvise_free_huge_pmd means the following processing for the PMD should
>> be skipped, while return 0 means the following processing is still
>> needed.  So the function should return 0 only if the THP is split
>> successfully or the PMD is not trans huge.  But the pmd_trans_unstable
>> after madvise_free_huge_pmd guarantee the following processing will be
>> skipped for huge PMD.  So current code can run properly.  But if my
>> understanding were correct, we can clean up return code of
>> madvise_free_huge_pmd accordingly.
>
> I like your clean up. Just a minor comment below.
>
>> 
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> ---
>>  mm/huge_memory.c | 7 +------
>>  1 file changed, 1 insertion(+), 6 deletions(-)
>> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 2ad52d5..64dc95d 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>
> First of all, let's change ret from int to bool.
> And then, add description in the function entry.

Yes.  bool looks better than int.

> /*
>  * Return true if we do MADV_FREE successfully on entire pmd page.
>  * Otherwise, return false.
>  */

This way, we need to return false if we failed to split huge page, this
will cause unnecessary pmd_trans_unstable check.  How about to change
the comments to

/*
 * Return true if we finished processing entire pmd page and needn't
 * fall back pte processing.  Otherwise, return false.
 */

Best Regards,
Huang, Ying

> And do not set to 1 if it is huge_zero_pmd but just goto out to
> return false.
>
> Thanks!
>
>> @@ -1655,14 +1655,9 @@ int madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>>  	if (next - addr != HPAGE_PMD_SIZE) {
>>  		get_page(page);
>>  		spin_unlock(ptl);
>> -		if (split_huge_page(page)) {
>> -			put_page(page);
>> -			unlock_page(page);
>> -			goto out_unlocked;
>> -		}
>> +		split_huge_page(page);
>>  		put_page(page);
>>  		unlock_page(page);
>> -		ret = 1;
>>  		goto out_unlocked;
>>  	}
>>  
>> -- 
>> 2.8.1
>> 
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