linux-kernel - Re: [PATCH v4 3/4] mm/page_reporting: Allow driver to specify reporting order

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Date:   Fri, 25 Jun 2021 16:04:38 +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 3/4] mm/page_reporting: Allow driver to specify
 reporting order

On 6/25/21 3:48 PM, Michael S. Tsirkin wrote:
> On Fri, Jun 25, 2021 at 09:47:09AM +0800, Gavin Shan wrote:
>> The page reporting order (threshold) is sticky to @pageblock_order
>> by default. The page reporting can never be triggered because the
>> freeing page can't come up with a free area like that huge. The
>> situation becomes worse when the system memory becomes heavily
>> fragmented.
>>
>> For example, the following configurations are used on ARM64 when 64KB
>> base page size is enabled. In this specific case, the page reporting
>> won't be triggered until the freeing page comes up with a 512MB free
>> area. That's hard to be met, especially when the system memory becomes
>> heavily fragmented.
>>
>>     PAGE_SIZE:          64KB
>>     HPAGE_SIZE:         512MB
>>     pageblock_order:    13       (512MB)
>>     MAX_ORDER:          14
>>
>> This allows the drivers to specify the page reporting order when the
>> page reporting device is registered. It falls back to @pageblock_order
>> if it's not specified by the driver. The existing users (hv_balloon
>> and virtio_balloon) don't specify it and @pageblock_order is still
>> taken as their page reporting order. So this shouldn't introduce any
>> functional changes.
>>
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> Reviewed-by: Alexander Duyck <alexanderduyck@...com>
>> ---
>>   include/linux/page_reporting.h | 3 +++
>>   mm/page_reporting.c            | 6 ++++++
>>   2 files changed, 9 insertions(+)
>>
>> diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
>> index 3b99e0ec24f2..fe648dfa3a7c 100644
>> --- a/include/linux/page_reporting.h
>> +++ b/include/linux/page_reporting.h
>> @@ -18,6 +18,9 @@ struct page_reporting_dev_info {
>>   
>>   	/* Current state of page reporting */
>>   	atomic_t state;
>> +
>> +	/* Minimal order of page reporting */
>> +	unsigned int order;
>>   };
>>   
>>   /* Tear-down and bring-up for page reporting devices */
>> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
>> index 34bf4d26c2c4..382958eef8a9 100644
>> --- a/mm/page_reporting.c
>> +++ b/mm/page_reporting.c
>> @@ -329,6 +329,12 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
>>   		goto err_out;
>>   	}
>>   
>> +	/*
>> +	 * Update the page reporting order if it's specified by driver.
>> +	 * Otherwise, it falls back to @pageblock_order.
>> +	 */
>> +	page_reporting_order = prdev->order ? : pageblock_order;
>> +
> 
> Hmm. So on ARM achitectures with 64K pages, the command line parameter
> is silently ignored?
> 
> Isn't this a problem?
> 

It's fine as the command line parameter is used as debugging purpose.
Besides, it can be changed by writing to the following file:

    /sys/module/page_reporting/parameters/page_reporting_order


>>   	/* initialize state and work structures */
>>   	atomic_set(&prdev->state, PAGE_REPORTING_IDLE);
>>   	INIT_DELAYED_WORK(&prdev->work, &page_reporting_process);
>> -- 
>> 2.23.0

Thanks,
Gavin