linux-kernel - Re: [PATCH 08/20] blkio: Add support for dynamic creation of cfq

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Date:	Wed, 04 Nov 2009 11:01:51 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-kernel@...r.kernel.org, jens.axboe@...cle.com,
	nauman@...gle.com, dpshah@...gle.com, lizf@...fujitsu.com,
	ryov@...inux.co.jp, fernando@....ntt.co.jp, s-uchida@...jp.nec.com,
	taka@...inux.co.jp, guijianfeng@...fujitsu.com,
	balbir@...ux.vnet.ibm.com, righi.andrea@...il.com,
	m-ikeda@...jp.nec.com, akpm@...ux-foundation.org, riel@...hat.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 08/20] blkio: Add support for dynamic creation of cfq_groups

Vivek Goyal <vgoyal@...hat.com> writes:

> +	/* Do we need to take this reference */
> +	if (!css_tryget(&blkcg->css))
> +		return NULL;;

Interesting question.  It seems like, so long as you want to reference
that cgroup, you need a reference on it.  ->alloc will only give you
one, so as long as we have pointers to these squirreled away, we need to
bump the reference count.  Please take a look at this before your next
posting.

> +
> +	cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, key));
> +	if (cfqg || !create)
> +		goto done;
> +
> +	cfqg = kzalloc_node(sizeof(*cfqg), GFP_ATOMIC |  __GFP_ZERO,
> +					cfqd->queue->node);
Do you really have to OR in __GFP_ZERO for kzalloc?

> +	cfqg = cfq_find_alloc_cfqg(cfqd, cgroup, create);
> +	if (!cfqg && create)
> +		cfqg = &cfqd->root_group;

Hmm, is that really the behaviour you want?

Cheers,
Jeff
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