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

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Date:   Fri, 25 Jun 2021 01:48:42 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Gavin Shan <gshan@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        alexander.duyck@...il.com, david@...hat.com,
        akpm@...ux-foundation.org, anshuman.khandual@....com,
        catalin.marinas@....com, will@...nel.org, shan.gavin@...il.com
Subject: Re: [PATCH v4 3/4] mm/page_reporting: Allow driver to specify
 reporting order

On Fri, Jun 25, 2021 at 09:47:09AM +0800, Gavin Shan 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@...hat.com>
> Reviewed-by: Alexander Duyck <alexanderduyck@...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;
> +

Hmm. So on ARM achitectures with 64K pages, the command line parameter
is silently ignored?

Isn't this a problem?

>  	/* initialize state and work structures */
>  	atomic_set(&prdev->state, PAGE_REPORTING_IDLE);
>  	INIT_DELAYED_WORK(&prdev->work, &page_reporting_process);
> -- 
> 2.23.0