Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

[Barry Song <21cnbao@gmail.com>]

On Mon, Mar 11, 2024 at 11:01 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> Now that swap supports storing all mTHP sizes, avoid splitting large
> folios before swap-out. This benefits performance of the swap-out path
> by eliding split_folio_to_list(), which is expensive, and also sets us
> up for swapping in large folios in a future series.
>
> If the folio is partially mapped, we continue to split it since we want
> to avoid the extra IO overhead and storage of writing out pages
> uneccessarily.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  mm/vmscan.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index cf7d4cf47f1a..0ebec99e04c6 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>                                         if (!can_split_folio(folio, NULL))
>                                                 goto activate_locked;
>                                         /*
> -                                        * Split folios without a PMD map right
> -                                        * away. Chances are some or all of the
> -                                        * tail pages can be freed without IO.
> +                                        * Split partially mapped folios map
> +                                        * right away. Chances are some or all
> +                                        * of the tail pages can be freed
> +                                        * without IO.
>                                          */
> -                                       if (!folio_entire_mapcount(folio) &&
> +                                       if (!list_empty(&folio->_deferred_list) &&

Hi Ryan,
After reconsidering our previous discussion about PMD-mapped large
folios, I've pondered
the possibility of PMD-mapped Transparent Huge Pages (THPs) being
mapped by multiple
processes. In such a scenario, if one process decides to unmap a
portion of the folio while
others retain the entire mapping, it raises questions about how the
system should handle
this situation. Would the large folio be placed in a deferred list? If
so, splitting it might not
yield benefits, as neither I/O nor swap slots would increase in this
case by not splitting it.

Regarding PTE-mapped large folios, the absence of an indicator like
"entire_map" makes it
challenging to identify cases where the entire folio is mapped. Thus,
splitting seems to be
the only viable solution in such circumstances.

>                                             split_folio_to_list(folio,
>                                                                 folio_list))
>                                                 goto activate_locked;
> --
> 2.25.1

Thanks
Barry