Re: [PATCH v4 3/4] mm/page_reporting: Allow driver to specify reporting order

[Alexander Duyck <alexander.duyck@gmail.com>]

On Thu, Jun 24, 2021 at 4:46 PM Gavin Shan <gshan@redhat.com> wrote:
>
> The page reporting order (threshold) is sticky to @pageblock_order
> by default. The page reporting can never be triggered because the
> freeing page can't come up with a free area like that huge. The
> situation becomes worse when the system memory becomes heavily
> fragmented.
>
> For example, the following configurations are used on ARM64 when 64KB
> base page size is enabled. In this specific case, the page reporting
> won't be triggered until the freeing page comes up with a 512MB free
> area. That's hard to be met, especially when the system memory becomes
> heavily fragmented.
>
>    PAGE_SIZE:          64KB
>    HPAGE_SIZE:         512MB
>    pageblock_order:    13       (512MB)
>    MAX_ORDER:          14
>
> This allows the drivers to specify the page reporting order when the
> page reporting device is registered. It falls back to @pageblock_order
> if it's not specified by the driver. The existing users (hv_balloon
> and virtio_balloon) don't specify it and @pageblock_order is still
> taken as their page reporting order. So this shouldn't introduce any
> functional changes.
>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
> ---
>  include/linux/page_reporting.h | 3 +++
>  mm/page_reporting.c            | 6 ++++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
> index 3b99e0ec24f2..fe648dfa3a7c 100644
> --- a/include/linux/page_reporting.h
> +++ b/include/linux/page_reporting.h
> @@ -18,6 +18,9 @@ struct page_reporting_dev_info {
>
>         /* Current state of page reporting */
>         atomic_t state;
> +
> +       /* Minimal order of page reporting */
> +       unsigned int order;
>  };
>
>  /* Tear-down and bring-up for page reporting devices */
> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index 34bf4d26c2c4..382958eef8a9 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -329,6 +329,12 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
>                 goto err_out;
>         }
>
> +       /*
> +        * Update the page reporting order if it's specified by driver.
> +        * Otherwise, it falls back to @pageblock_order.
> +        */
> +       page_reporting_order = prdev->order ? : pageblock_order;
> +

An alternative to this would be to look at setting up some
comparisons. I might add another variable and do something like:
order = prdev->order ? : pageblock_order;
if (order < page_reporting_order)
    page_reporting_order = order;

You could essentially do something similar in the previous patch but
just use pageblock_order directly rather than having to add a local
variable.

That way if you need to still pull down the page reporting order you
can do so without prdev->order or pageblock_order overwriting the
value and pushing it back up.