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

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Date:   Fri, 25 Jun 2021 16:08:13 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     "Michael S. Tsirkin" <mst@...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 2/4] mm/page_reporting: Export reporting order as
 module parameter

On 6/25/21 3:53 PM, Michael S. Tsirkin wrote:
> 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@...hat.com>
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> Reviewed-by: Alexander Duyck <alexanderduyck@...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?
> 

Well, I guess it's fine as it's used for debugging purpose. I guess
it's mostly used by developers. Also, the value can be changed by
the module parameter in "/sys/module/page_reporting" either.

>> +
>>   	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.
> 

This function won't be run if page_reporting_order is more than (MAX_ORDER-1).

>>   
>>   	/*
>>   	 * 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
> 

Thanks,
Gavin