linux-kernel - Re: [PATCH] mm/zbud: init user ops only when it is needed

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Date:	Thu, 16 Oct 2014 14:48:42 +0900
From:	Heesub Shin <heesub.shin@...sung.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dan Streetman <ddstreet@...e.org>,
	Seth Jennings <sjennings@...iantweb.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Sunae Seo <sunae.seo@...sung.com>
Subject: Re: [PATCH] mm/zbud: init user ops only when it is needed

Hello,

On 10/16/2014 05:17 AM, Andrew Morton wrote:
> On Wed, 15 Oct 2014 19:00:43 +0900 Heesub Shin <heesub.shin@...sung.com> wrote:
>
>> When zbud is initialized through the zpool wrapper, pool->ops which
>> points to user-defined operations is always set regardless of whether it
>> is specified from the upper layer. This causes zbud_reclaim_page() to
>> iterate its loop for evicting pool pages out without any gain.
>>
>> This patch sets the user-defined ops only when it is needed, so that
>> zbud_reclaim_page() can bail out the reclamation loop earlier if there
>> is no user-defined operations specified.
>
> Which callsite is calling zbud_zpool_create(..., NULL)?

Currently nowhere. zswap is the only user of zbud and always passes a 
pointer to user-defined operation on pool creation. In addition, there 
may be less possibility that pool shrinking is requested by users who 
did not provide the user-defined ops. So, we may not need to worry much 
about what I wrote in the changelog. However, it is definitely weird to 
pass an argument, zpool_ops, which even will not be referenced by 
zbud_zpool_create(). Above all, it would be more useful to avoid the 
possibility in the future rather than just ignoring it.

regards,
heesub

>
>> ...
>> --- a/mm/zbud.c
>> +++ b/mm/zbud.c
>> @@ -132,7 +132,7 @@ static struct zbud_ops zbud_zpool_ops = {
>>
>>   static void *zbud_zpool_create(gfp_t gfp, struct zpool_ops *zpool_ops)
>>   {
>> -	return zbud_create_pool(gfp, &zbud_zpool_ops);
>> +	return zbud_create_pool(gfp, zpool_ops ? &zbud_zpool_ops : NULL);
>>   }
>>
>>   static void zbud_zpool_destroy(void *pool)
>
>
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