Re: [PATCH V2 2/3] Drivers: hv: balloon: Support 2M page allocations for ballooning

[Michal Hocko <mhocko@suse.cz>]

On Mon 18-03-13 13:51:37, K. Y. Srinivasan wrote:
> On Hyper-V it will be very efficient to use 2M allocations in the guest as this
> makes the ballooning protocol with the host that much more efficient. Hyper-V
> uses page ranges (start pfn : number of pages) to specify memory being moved
> around and with 2M pages this encoding can be very efficient. However, when
> memory is returned to the guest, the host does not guarantee any granularity.
> To deal with this issue, split the page soon after a successful 2M allocation
> so that this memory can potentially be freed as 4K pages.

How many pages are requested usually?

> If 2M allocations fail, we revert to 4K allocations.
> 
> In this version of the patch, based on the feedback from Michal Hocko
> <mhocko@suse.cz>, I have added some additional commentary to the patch
> description. 
> 
> Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>

I am not going to ack the patch because I am still not entirely
convinced that big allocations are worth it. But that is up to you and
hyper-V users.

> ---
>  drivers/hv/hv_balloon.c |   18 ++++++++++++++++--
>  1 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 2cf7d4e..71655b4 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -997,6 +997,14 @@ static int  alloc_balloon_pages(struct hv_dynmem_device *dm, int num_pages,
>  
>  		dm->num_pages_ballooned += alloc_unit;
>  
> +		/*
> +		 * If we allocatted 2M pages; split them so we
> +		 * can free them in any order we get.
> +		 */
> +
> +		if (alloc_unit != 1)
> +			split_page(pg, get_order(alloc_unit << PAGE_SHIFT));
> +
>  		bl_resp->range_count++;
>  		bl_resp->range_array[i].finfo.start_page =
>  			page_to_pfn(pg);

I would suggest also using __GFP_NO_KSWAPD (or basically use
GFP_TRANSHUGE for alloc_unit>0) for the allocation to be as least
disruptive as possible.

> @@ -1023,9 +1031,10 @@ static void balloon_up(struct work_struct *dummy)
>  
>  
>  	/*
> -	 * Currently, we only support 4k allocations.
> +	 * We will attempt 2M allocations. However, if we fail to
> +	 * allocate 2M chunks, we will go back to 4k allocations.
>  	 */
> -	alloc_unit = 1;
> +	alloc_unit = 512;
>  
>  	while (!done) {
>  		bl_resp = (struct dm_balloon_response *)send_buffer;
> @@ -1041,6 +1050,11 @@ static void balloon_up(struct work_struct *dummy)
>  						bl_resp, alloc_unit,
>  						 &alloc_error);
>  

You should handle alloc_balloon_pages returns 0 && !alloc_error which
happens when num_pages < alloc_unit.

> +		if ((alloc_error) && (alloc_unit != 1)) {
> +			alloc_unit = 1;
> +			continue;
> +		}
> +
>  		if ((alloc_error) || (num_ballooned == num_pages)) {
>  			bl_resp->more_pages = 0;
>  			done = true;
> -- 
> 1.7.4.1
> 
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-- 
Michal Hocko
SUSE Labs