Re: [PATCH v4 2/4] mm/page_reporting: Export reporting order as module parameter

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jun 25, 2021 at 09:47:08AM +0800, Gavin Shan wrote:
> The macro PAGE_REPORTING_MIN_ORDER is defined as the page reporting
> threshold. It can't be adjusted at runtime.
> 
> This introduces a variable (@page_reporting_order) to replace the
> marcro (PAGE_REPORTING_MIN_ORDER). MAX_ORDER is assigned to it initially,
> meaning the page reporting is disabled. It will be specified by driver
> if valid one is provided. Otherwise, it will fall back to @pageblock_order.
> It's also exported so that the page reporting order can be adjusted at
> runtime.
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
>  mm/page_reporting.c                             | 9 +++++++--
>  mm/page_reporting.h                             | 5 ++---
>  3 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index cb89dbdedc46..566c4b9af3cd 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3566,6 +3566,12 @@
>  			off: turn off poisoning (default)
>  			on: turn on poisoning
>  
> +	page_reporting.page_reporting_order=
> +			[KNL] Minimal page reporting order
> +			Format: <integer>
> +			Adjust the minimal page reporting order. The page
> +			reporting is disabled when it exceeds (MAX_ORDER-1).

Which the admin knows how? Run grep in the kernel source?

> +
>  	panic=		[KNL] Kernel behaviour on panic: delay <timeout>
>  			timeout > 0: seconds before rebooting
>  			timeout = 0: wait forever
> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index df9c5054e1b4..34bf4d26c2c4 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -4,12 +4,17 @@
>  #include <linux/page_reporting.h>
>  #include <linux/gfp.h>
>  #include <linux/export.h>
> +#include <linux/module.h>
>  #include <linux/delay.h>
>  #include <linux/scatterlist.h>
>  
>  #include "page_reporting.h"
>  #include "internal.h"
>  
> +unsigned int page_reporting_order = MAX_ORDER;
> +module_param(page_reporting_order, uint, 0644);
> +MODULE_PARM_DESC(page_reporting_order, "Set page reporting order");
> +
>  #define PAGE_REPORTING_DELAY	(2 * HZ)
>  static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly;
>  
> @@ -229,7 +234,7 @@ page_reporting_process_zone(struct page_reporting_dev_info *prdev,
>  
>  	/* Generate minimum watermark to be able to guarantee progress */
>  	watermark = low_wmark_pages(zone) +
> -		    (PAGE_REPORTING_CAPACITY << PAGE_REPORTING_MIN_ORDER);
> +		    (PAGE_REPORTING_CAPACITY << page_reporting_order);


Looks like this makes it easy to trigger undefined behaviour. Just use
any value > 31.

>  
>  	/*
>  	 * Cancel request if insufficient free memory or if we failed
> @@ -239,7 +244,7 @@ page_reporting_process_zone(struct page_reporting_dev_info *prdev,
>  		return err;
>  
>  	/* Process each free list starting from lowest order/mt */
> -	for (order = PAGE_REPORTING_MIN_ORDER; order < MAX_ORDER; order++) {
> +	for (order = page_reporting_order; order < MAX_ORDER; order++) {
>  		for (mt = 0; mt < MIGRATE_TYPES; mt++) {
>  			/* We do not pull pages from the isolate free list */
>  			if (is_migrate_isolate(mt))
> diff --git a/mm/page_reporting.h b/mm/page_reporting.h
> index 2c385dd4ddbd..c51dbc228b94 100644
> --- a/mm/page_reporting.h
> +++ b/mm/page_reporting.h
> @@ -10,10 +10,9 @@
>  #include <linux/pgtable.h>
>  #include <linux/scatterlist.h>
>  
> -#define PAGE_REPORTING_MIN_ORDER	pageblock_order
> -
>  #ifdef CONFIG_PAGE_REPORTING
>  DECLARE_STATIC_KEY_FALSE(page_reporting_enabled);
> +extern unsigned int page_reporting_order;
>  void __page_reporting_notify(void);
>  
>  static inline bool page_reported(struct page *page)
> @@ -38,7 +37,7 @@ static inline void page_reporting_notify_free(unsigned int order)
>  		return;
>  
>  	/* Determine if we have crossed reporting threshold */
> -	if (order < PAGE_REPORTING_MIN_ORDER)
> +	if (order < page_reporting_order)
>  		return;
>  
>  	/* This will add a few cycles, but should be called infrequently */
> -- 
> 2.23.0