linux-kernel - Re: [PATCHv2 1/9] staging: zsmalloc: add gfp flags to zs_create

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Date:	Fri, 25 Jan 2013 09:07:47 -0600
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv2 1/9] staging: zsmalloc: add gfp flags to zs_create_pool

On 01/24/2013 07:33 PM, Minchan Kim wrote:
> Hi Seth, frontswap guys
> 
> On Tue, Jan 8, 2013 at 5:24 AM, Seth Jennings
> <sjenning@...ux.vnet.ibm.com> wrote:
>> zs_create_pool() currently takes a gfp flags argument
>> that is used when growing the memory pool.  However
>> it is not used in allocating the metadata for the pool
>> itself.  That is currently hardcoded to GFP_KERNEL.
>>
>> zswap calls zs_create_pool() at swapon time which is done
>> in atomic context, resulting in a "might sleep" warning.
> 
> I didn't review this all series, really sorry but totday I saw Nitin
> added Acked-by so I'm afraid Greg might get it under my radar. I'm not
> strong against but I would like know why we should call frontswap_init
> under swap_lock? Is there special reason?

The call stack is:

SYSCALL_DEFINE2(swapon.. <-- swapon_mutex taken here
enable_swap_info() <-- swap_lock taken here
frontswap_init()
__frontswap_init()
zswap_frontswap_init()
zs_create_pool()

It isn't entirely clear to me why frontswap_init() is called under
lock.  Then again, I'm not entirely sure what the swap_lock protects.
 There are no comments near the swap_lock definition to tell me.

I would guess that the intent is to block any writes to the swap
device until frontswap_init() has completed.

Dan care to weigh in?

Seth

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