Re: [RFC PATCH v2 9/9] docs: mm: update THP swapin counter descriptions

[Nhat Pham]

On Fri, May 29, 2026 at 5:19 AM fujunjie <fujunjie1@xxxxxx> wrote:
>
> The THP swapin counter descriptions still describe large swapin as
> coming only from non-zswap swap devices. Update them now that
> zswap-backed large folio swapin can also increment swpin.
>
> Also describe policy and backend rejection as swpin_fallback cases,
> since speculative zswap large swapin can intentionally fall back before
> doing large IO.
>
> Signed-off-by: fujunjie <fujunjie1@xxxxxx>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 23f8d13c2629..59b7a0d09243 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -667,13 +667,14 @@ zswpout
>         piece without splitting.
>
>  swpin
> -       is incremented every time a huge page is swapped in from a non-zswap
> -       swap device in one piece.
> +       is incremented every time a huge page is swapped in from swap or
> +       zswap in one piece.
>
>  swpin_fallback
> -       is incremented if swapin fails to allocate or charge a huge page
> -       and instead falls back to using huge pages with lower orders or
> -       small pages.
> +       is incremented if swapin cannot use a huge page and instead falls
> +       back to using huge pages with lower orders or small pages. This can
> +       happen because allocation or charging fails, or because policy or
> +       backend state rejects a speculative large swapin.

I think we should add separate zswpin and zswpin fallback counter for
THP rather than overloading swpin. We already do that for zswpout vs
swpout.