Re: [PATCHv2 4/8] khugepaged: Drain LRU add pagevec after swapin

[Yang Shi]

On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
> __collapse_huge_page_isolate() may fail due to extra pin in the LRU add
> pagevec. It's petty common for swapin case: we swap in pages just to
> fail due to the extra pin.
>
> Drain LRU add pagevec on sucessfull swapin.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>   mm/khugepaged.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index fdc10ffde1ca..57ff287caf6b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -940,6 +940,11 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
>   	}
>   	vmf.pte--;
>   	pte_unmap(vmf.pte);
> +
> +	/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
> +	if (swapped_in)
> +		lru_add_drain();

There is already lru_add_drain() called in swap readahead path, please 
see swap_vma_readahead() and swap_cluster_readahead().

The extra call to draining lru seems unnecessary.

> +
>   	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 1);
>   	return true;
>   }