Re: [RFC PATCH 3/3] mm: support free hugepage pre zero out

[David Hildenbrand]

On 22.12.20 08:49, Liang Li wrote:
> This patch add support of pre zero out free hugepage, we can use
> this feature to speed up page population and page fault handing.
> 
> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>  
> Cc: Michal Hocko <mhocko@xxxxxxxx> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Liang Li <liliang324@xxxxxxxxx>
> Signed-off-by: Liang Li <liliangleo@xxxxxxxxxxxxxx>
> ---
>  mm/page_prezero.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/mm/page_prezero.c b/mm/page_prezero.c
> index c8ce720bfc54..dff4e0adf402 100644
> --- a/mm/page_prezero.c
> +++ b/mm/page_prezero.c
> @@ -26,6 +26,7 @@ static unsigned long delay_millisecs = 1000;
>  static unsigned long zeropage_enable __read_mostly;
>  static DEFINE_MUTEX(kzeropaged_mutex);
>  static struct page_reporting_dev_info zero_page_dev_info;
> +static struct page_reporting_dev_info zero_hugepage_dev_info;
>  
>  inline void clear_zero_page_flag(struct page *page, int order)
>  {
> @@ -69,9 +70,17 @@ static int start_kzeropaged(void)
>  		zero_page_dev_info.delay_jiffies = msecs_to_jiffies(delay_millisecs);
>  
>  		err = page_reporting_register(&zero_page_dev_info);
> +
> +		zero_hugepage_dev_info.report = zero_free_pages;
> +		zero_hugepage_dev_info.mini_order = mini_page_order;
> +		zero_hugepage_dev_info.batch_size = batch_size;
> +		zero_hugepage_dev_info.delay_jiffies = msecs_to_jiffies(delay_millisecs);
> +
> +		err |= hugepage_reporting_register(&zero_hugepage_dev_info);
>  		pr_info("Zero page enabled\n");
>  	} else {
>  		page_reporting_unregister(&zero_page_dev_info);
> +		hugepage_reporting_unregister(&zero_hugepage_dev_info);
>  		pr_info("Zero page disabled\n");
>  	}
>  
> @@ -90,7 +99,15 @@ static int restart_kzeropaged(void)
>  		zero_page_dev_info.batch_size = batch_size;
>  		zero_page_dev_info.delay_jiffies = msecs_to_jiffies(delay_millisecs);
>  
> +		hugepage_reporting_unregister(&zero_hugepage_dev_info);
> +
> +		zero_hugepage_dev_info.report = zero_free_pages;
> +		zero_hugepage_dev_info.mini_order = mini_page_order;
> +		zero_hugepage_dev_info.batch_size = batch_size;
> +		zero_hugepage_dev_info.delay_jiffies = msecs_to_jiffies(delay_millisecs);
> +
>  		err = page_reporting_register(&zero_page_dev_info);
> +		err |= hugepage_reporting_register(&zero_hugepage_dev_info);
>  		pr_info("Zero page enabled\n");
>  	}
>  
> 

Free page reporting in virtio-balloon doesn't give you any guarantees
regarding zeroing of pages. Take a look at the QEMU implementation -
e.g., with vfio all reports are simply ignored.

Also, I am not sure if mangling such details ("zeroing of pages") into
the page reporting infrastructure is a good idea.

-- 
Thanks,

David / dhildenb