linux-kernel - Re: [PATCH v8, part3 10/14] mm: use a dedicated lock to protect totalram_pages and zone->managed

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Date:	Wed, 12 Jun 2013 10:17:53 +0800
From:	Jiang Liu <liuj97@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Jiang Liu <jiang.liu@...wei.com>,
	David Rientjes <rientjes@...gle.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>,
	James Bottomley <james.bottomley@...senpartnership.com>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	David Howells <dhowells@...hat.com>,
	Mark Salter <msalter@...hat.com>,
	Jianguo Wu <wujianguo@...wei.com>, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v8, part3 10/14] mm: use a dedicated lock to protect totalram_pages
 and zone->managed_pages

On Wed 12 Jun 2013 04:00:42 AM CST, Andrew Morton wrote:
> On Sun, 26 May 2013 21:38:38 +0800 Jiang Liu <liuj97@...il.com> wrote:
>
>> Currently lock_memory_hotplug()/unlock_memory_hotplug() are used to
>> protect totalram_pages and zone->managed_pages. Other than the memory
>> hotplug driver, totalram_pages and zone->managed_pages may also be
>> modified at runtime by other drivers, such as Xen balloon,
>> virtio_balloon etc. For those cases, memory hotplug lock is a little
>> too heavy, so introduce a dedicated lock to protect totalram_pages
>> and zone->managed_pages.
>>
>> Now we have a simplified locking rules totalram_pages and
>> zone->managed_pages as:
>> 1) no locking for read accesses because they are unsigned long.
>> 2) no locking for write accesses at boot time in single-threaded context.
>> 3) serialize write accesses at runtime by acquiring the dedicated
>>    managed_page_count_lock.
>>
>> Also adjust zone->managed_pages when freeing reserved pages into the
>> buddy system, to keep totalram_pages and zone->managed_pages in
>> consistence.
>>
>> ...
>>
>> +void adjust_managed_page_count(struct page *page, long count)
>> +{
>> +	spin_lock(&managed_page_count_lock);
>> +	page_zone(page)->managed_pages += count;
>> +	totalram_pages += count;
>> +	spin_unlock(&managed_page_count_lock);
>> +}
>> +EXPORT_SYMBOL(adjust_managed_page_count);
>
> This is exported to modules but there are no modular callers at this
> time.
>
> I assume this was done for some forthcoming xen/virtio_balloon/etc
> patches?  If so, it would be better to avoid adding the export until it
> is actually needed.
Hi Andrew,
     adjust_managed_page_count() will be used by virtio_balloon and xen 
balloon
drivers. Grep mmots tree:
drivers/virtio/virtio_balloon.c:		adjust_managed_page_count(page, -1);
drivers/virtio/virtio_balloon.c:		adjust_managed_page_count(page, 1);
drivers/xen/balloon.c:	adjust_managed_page_count(page, -1);
drivers/xen/balloon.c:	adjust_managed_page_count(page, 1);

So if we un-export it in part3, we need to export in part4 again.
Regards!
Gerry

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