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Date: Mon, 18 Feb 2013 13:59:25 -0800 (PST)
From: Dan Magenheimer <dan.magenheimer@oracle.com>
To: Seth Jennings <sjenning@linux.vnet.ibm.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/
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> From: Seth Jennings [mailto:sjenning@linux.vnet.ibm.com]
> Subject: Re: [PATCHv5 4/8] zswap: add to mm/
> 
> 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:
> >>>> + * 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.

Hmmm... I wonder if exporting these might be the best solution
as it (unnecessarily?) exposes some swap subsystem internals.
I wonder if a small change to read_swap_cache_async might
be more acceptable.