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Date: Mon, 18 Feb 2013 14:39:38 -0600
From: Seth Jennings <sjenning@linux.vnet.ibm.com>
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To: Dan Magenheimer <dan.magenheimer@oracle.com>
CC: Ric Mason <ric.masonn@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Nitin Gupta <ngupta@vflare.org>, Minchan Kim <minchan@kernel.org>,
        Konrad Wilk <konrad.wilk@oracle.com>,
        Robert Jennings <rcj@linux.vnet.ibm.com>,
        Jenifer Hopper <jhopper@us.ibm.com>, Mel Gorman <mgorman@suse.de>,
        Johannes Weiner <jweiner@redhat.com>, Rik van Riel <riel@redhat.com>,
        Larry Woodman <lwoodman@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Dave Hansen <dave@linux.vnet.ibm.com>, Joe Perches <joe@perches.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        devel@driverdev.osuosl.org
Subject: Re: [PATCHv5 4/8] zswap: add to mm/
References: <1360780731-11708-1-git-send-email-sjenning@linux.vnet.ibm.com> <1360780731-11708-5-git-send-email-sjenning@linux.vnet.ibm.com> <511F0536.5030802@gmail.com> <51227FDA.7040000@linux.vnet.ibm.com> <0fb2af92-575f-4f5d-a115-829a3cf035e5@default>
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On 02/18/2013 01:55 PM, Dan Magenheimer wrote:
>> From: Seth Jennings [mailto:sjenning@linux.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