linux-kernel - Re: [PATCHv5 4/8] zswap: add to mm/

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Message-ID: <5122918A.8090307@linux.vnet.ibm.com>
Date:	Mon, 18 Feb 2013 14:39:38 -0600
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Ric Mason <ric.masonn@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Wilk <konrad.wilk@...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>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Joe Perches <joe@...ches.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv5 4/8] zswap: add to mm/

On 02/18/2013 01:55 PM, Dan Magenheimer wrote:
>> From: Seth Jennings [mailto:sjenning@...ux.vnet.ibm.com]
>> Subject: Re: [PATCHv5 4/8] zswap: add to mm/
>>
>> On 02/15/2013 10:04 PM, Ric Mason wrote:
>>> On 02/14/2013 02:38 AM, Seth Jennings wrote:
>> <snip>
>>>> + * The statistics below are not protected from concurrent access for
>>>> + * performance reasons so they may not be a 100% accurate.  However,
>>>> + * the do provide useful information on roughly how many times a
>>>
>>> s/the/they
>>
>> Ah yes, thanks :)
>>
>>>
>>>> + * certain event is occurring.
>>>> +*/
>>>> +static u64 zswap_pool_limit_hit;
>>>> +static u64 zswap_reject_compress_poor;
>>>> +static u64 zswap_reject_zsmalloc_fail;
>>>> +static u64 zswap_reject_kmemcache_fail;
>>>> +static u64 zswap_duplicate_entry;
>>>> +
>>>> +/*********************************
>>>> +* tunables
>>>> +**********************************/
>>>> +/* Enable/disable zswap (disabled by default, fixed at boot for
>>>> now) */
>>>> +static bool zswap_enabled;
>>>> +module_param_named(enabled, zswap_enabled, bool, 0);
>>>
>>> please document in Documentation/kernel-parameters.txt.
>>
>> Will do.
> 
> Is that a good idea?  Konrad's frontswap/cleancache patches
> to fix frontswap/cleancache initialization so that backends
> can be built/loaded as modules may be merged for 3.9.
> AFAIK, module parameters are not included in kernel-parameters.txt.

This is true.  However, the frontswap/cleancache init stuff isn't the
only reason zswap is built-in only.  The writeback code depends on
non-exported kernel symbols:

swapcache_free
__swap_writepage
__add_to_swap_cache
swapcache_prepare
swapper_space
end_swap_bio_write

I know a fix is as trivial as exporting them, but I didn't want to
take on that debate right now.

Seth

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