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

[Minchan Kim]

Hi,

On Thu, Jun 16, 2016 at 08:03:54PM -0700, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> 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@xxxxxxxxx>
> ---
>  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.

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

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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