Re: [PATCH mm-unstable 1/4] mm/khugepaged: Use minimal bits to store num page < HPAGE_PMD_NR

[Yang Shi <shy828301@gmail.com>]

On Wed, Jul 20, 2022 at 7:06 AM Zach O'Keefe <zokeefe@google.com> wrote:
>
> Minimally, node_load[] entries just need to be able to hold the maximum
> value of HPAGE_PMD_NR, which is compile-time defined per-arch based on
> PMD_SHIFT and PAGE_SHIFT.  node_load[] is only written either via memset(),
> or with via post-increment. struct collapse_control may be allocated
> via kmalloc() in other collapse contexts, and MAX_NUMNODES may be
> arbitrarily large. #define the underlying type of node_load[] based off
> HPAGE_PMD_NR to avoid excessive memory allocated for this struct.
>
> Fixes: 3b07f3bb225a ("mm/khugepaged: add struct collapse_control")
> Link: https://lore.kernel.org/linux-mm/Ys2CeIm%2FQmQwWh9a@google.com/
> Signed-off-by: Zach O'Keefe <zokeefe@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
>  mm/khugepaged.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 69990dacde14..ecd28bfeab60 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -92,8 +92,11 @@ struct collapse_control {
>         bool is_khugepaged;
>
>         /* Num pages scanned per node */
> -       int node_load[MAX_NUMNODES];
> -
> +#if HPAGE_PMD_ORDER < 16
> +       u16 node_load[MAX_NUMNODES];
> +#else
> +       u32 node_load[MAX_NUMNODES];
> +#endif
>         /* Last target selected in hpage_collapse_find_target_node() */
>         int last_target_node;
>  };
> --
> 2.37.0.170.g444d1eabd0-goog
>