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

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Date:	Mon, 18 Feb 2013 13:59:25 -0800 (PST)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.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/

> From: Seth Jennings [mailto:sjenning@...ux.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@...ux.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.
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