RE: [PATCH v2 2/2] hv_balloon: Add support for configurable order free page reporting

["Michael Kelley (LINUX)" <mikelley@microsoft.com>]

From: Shradha Gupta <shradhagupta@linux.microsoft.com> Sent: Thursday, September 29, 2022 11:02 PM
> 
> Newer versions of Hyper-V allow reporting unused guest pages in chunks
> smaller than 2 Mbytes.  Using smaller chunks allows reporting more
> unused guest pages, but with increased overhead in the finding the
> small chunks.  To make this tradeoff configurable, use the existing
> page_reporting_order module parameter to control the reporting order.
> Drop and refine checks that restricted the minimun page reporting order
> to 2Mbytes size pages. Add appropriate checks to make sure the
> underlying Hyper-V versions support cold discard hints of any order
> (and not just starting from 9)
> 
> Signed-off-by: Shradha Gupta <shradhagupta@linux.microsoft.com>
> ---
>  drivers/hv/hv_balloon.c | 94 ++++++++++++++++++++++++++++++++---------
>  1 file changed, 73 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index fdf6decacf06..7088ed056e50 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -469,12 +469,16 @@ static bool do_hot_add;
>   * the specified number of seconds.
>   */
>  static uint pressure_report_delay = 45;
> +extern unsigned int page_reporting_order;
> +#define HV_MAX_FAILURES	2
> 
>  /*
>   * The last time we posted a pressure report to host.
>   */
>  static unsigned long last_post_time;
> 
> +static int hv_hypercall_multi_failure;
> +
>  module_param(hot_add, bool, (S_IRUGO | S_IWUSR));
>  MODULE_PARM_DESC(hot_add, "If set attempt memory hot_add");
> 
> @@ -579,6 +583,10 @@ static struct hv_dynmem_device dm_device;
> 
>  static void post_status(struct hv_dynmem_device *dm);
> 
> +static void enable_page_reporting(void);
> +
> +static void disable_page_reporting(void);
> +
>  #ifdef CONFIG_MEMORY_HOTPLUG
>  static inline bool has_pfn_is_backed(struct hv_hotadd_state *has,
>  				     unsigned long pfn)
> @@ -1418,6 +1426,18 @@ static int dm_thread_func(void *dm_dev)
>  		 */
>  		reinit_completion(&dm_device.config_event);
>  		post_status(dm);
> +		/*
> +		 * disable free page reporting if multiple hypercall
> +		 * failure flag set. It is not done in the page_reporting
> +		 * callback context as that causes a deadlock between
> +		 * page_reporting_process() and page_reporting_unregister()
> +		 */
> +		if (hv_hypercall_multi_failure >= HV_MAX_FAILURES) {
> +			pr_err("Multiple failures in cold memory discard hypercall, disabling page reporting\n");
> +			disable_page_reporting();
> +			/* Reset the flag after disabling reporting */
> +			hv_hypercall_multi_failure = 0;
> +		}
>  	}
> 
>  	return 0;
> @@ -1593,20 +1613,20 @@ static void balloon_onchannelcallback(void *context)
> 
>  }
> 
> -/* Hyper-V only supports reporting 2MB pages or higher */
> -#define HV_MIN_PAGE_REPORTING_ORDER	9
> -#define HV_MIN_PAGE_REPORTING_LEN (HV_HYP_PAGE_SIZE <<
> HV_MIN_PAGE_REPORTING_ORDER)
> +#define HV_LARGE_REPORTING_ORDER	9
> +#define HV_LARGE_REPORTING_LEN (HV_HYP_PAGE_SIZE << \
> +		HV_LARGE_REPORTING_ORDER)
>  static int hv_free_page_report(struct page_reporting_dev_info *pr_dev_info,
>  		    struct scatterlist *sgl, unsigned int nents)
>  {
>  	unsigned long flags;
>  	struct hv_memory_hint *hint;
> -	int i;
> +	int i, order;
>  	u64 status;
>  	struct scatterlist *sg;
> 
>  	WARN_ON_ONCE(nents > HV_MEMORY_HINT_MAX_GPA_PAGE_RANGES);
> -	WARN_ON_ONCE(sgl->length < HV_MIN_PAGE_REPORTING_LEN);
> +	WARN_ON_ONCE(sgl->length < (HV_HYP_PAGE_SIZE << page_reporting_order));
>  	local_irq_save(flags);
>  	hint = *(struct hv_memory_hint **)this_cpu_ptr(hyperv_pcpu_input_arg);
>  	if (!hint) {
> @@ -1621,21 +1641,53 @@ static int hv_free_page_report(struct page_reporting_dev_info *pr_dev_info,
> 
>  		range = &hint->ranges[i];
>  		range->address_space = 0;
> -		/* page reporting only reports 2MB pages or higher */
> -		range->page.largepage = 1;
> -		range->page.additional_pages =
> -			(sg->length / HV_MIN_PAGE_REPORTING_LEN) - 1;
> -		range->page_size = HV_GPA_PAGE_RANGE_PAGE_SIZE_2MB;
> -		range->base_large_pfn =
> -			page_to_hvpfn(sg_page(sg)) >> HV_MIN_PAGE_REPORTING_ORDER;
> +		order = get_order(sg->length);
> +		/*
> +		 * Hyper-V expects the additional_pages field in the units
> +		 * of one of these 3 sizes, 4Kbytes, 2Mbytes or 1Gbytes.
> +		 * This is dictated by the values of the fields page.largesize
> +		 * and page_size.
> +		 * This code however, only uses 4Kbytes and 2Mbytes units
> +		 * and not 1Gbytes unit.
> +		 */
> +
> +		/* page reporting for pages 2MB or higher */
> +		if (order >= HV_LARGE_REPORTING_ORDER ) {
> +			range->page.largepage = 1;
> +			range->page_size = HV_GPA_PAGE_RANGE_PAGE_SIZE_2MB;
> +			range->base_large_pfn = page_to_hvpfn(
> +					sg_page(sg)) >> HV_LARGE_REPORTING_ORDER;
> +			range->page.additional_pages =
> +				(sg->length / HV_LARGE_REPORTING_LEN) - 1;
> +		} else {
> +			/* Page reporting for pages below 2MB */
> +			range->page.basepfn = page_to_hvpfn(sg_page(sg));
> +			range->page.largepage = false;
> +			range->page.additional_pages =
> +				(sg->length / HV_HYP_PAGE_SIZE) - 1;
> +		}
> +
>  	}
> 
>  	status = hv_do_rep_hypercall(HV_EXT_CALL_MEMORY_HEAT_HINT, nents, 0,
>  				     hint, NULL);
>  	local_irq_restore(flags);
> -	if ((status & HV_HYPERCALL_RESULT_MASK) != HV_STATUS_SUCCESS) {
> +	if (!hv_result_success(status)) {
> +
>  		pr_err("Cold memory discard hypercall failed with status %llx\n",
> -			status);
> +				status);
> +		if (hv_hypercall_multi_failure > 0)
> +			hv_hypercall_multi_failure++;
> +
> +		if (hv_result(status) == HV_STATUS_INVALID_PARAMETER) {
> +			pr_err("Underlying Hyper-V does not support order less than 9. Hypercall failed\n");
> +			pr_err("Defaulting to page_reporting_order %d\n",
> +					pageblock_order);
> +			page_reporting_order = pageblock_order;
> +			hv_hypercall_multi_failure++;
> +			return -EINVAL;
> +		}
> +
>  		return -EINVAL;
>  	}
> 
> @@ -1646,12 +1698,6 @@ static void enable_page_reporting(void)
>  {
>  	int ret;
> 
> -	/* Essentially, validating 'PAGE_REPORTING_MIN_ORDER' is big enough. */
> -	if (pageblock_order < HV_MIN_PAGE_REPORTING_ORDER) {
> -		pr_debug("Cold memory discard is only supported on 2MB pages and above\n");
> -		return;
> -	}
> -
>  	if (!hv_query_ext_cap(HV_EXT_CAPABILITY_MEMORY_COLD_DISCARD_HINT)) {
>  		pr_debug("Cold memory discard hint not supported by Hyper-V\n");
>  		return;
> @@ -1659,12 +1705,18 @@ static void enable_page_reporting(void)
> 
>  	BUILD_BUG_ON(PAGE_REPORTING_CAPACITY > HV_MEMORY_HINT_MAX_GPA_PAGE_RANGES);
>  	dm_device.pr_dev_info.report = hv_free_page_report;
> +	/*
> +	 * We let the page_reporting_order parameter decide the order
> +	 * in the page_reporting code
> +	 */
> +	dm_device.pr_dev_info.order = 0;
>  	ret = page_reporting_register(&dm_device.pr_dev_info);
>  	if (ret < 0) {
>  		dm_device.pr_dev_info.report = NULL;
>  		pr_err("Failed to enable cold memory discard: %d\n", ret);
>  	} else {
> -		pr_info("Cold memory discard hint enabled\n");
> +		pr_info("Cold memory discard hint enabled with order %d\n",
> +				page_reporting_order);
>  	}
>  }
> 
> --
> 2.37.2

Reviewed-by: Michael Kelley <mikelley@microsoft.com>