linux-kernel - RE: [PATCHv11 3/4] zswap: add to mm/

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Date:	Wed, 15 May 2013 13:55:44 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Dave Hansen <dave@...1.net>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Cc:	Konrad Wilk <konrad.wilk@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	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>,
	Joe Perches <joe@...ches.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Hugh Dickens <hughd@...gle.com>,
	Paul Mackerras <paulus@...ba.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: RE: [PATCHv11 3/4] zswap: add to mm/

> From: Dave Hansen [mailto:dave@...1.net]
> Sent: Wednesday, May 15, 2013 2:24 PM
> To: Seth Jennings
> Cc: Konrad Rzeszutek Wilk; Dan Magenheimer; Andrew Morton; Greg Kroah-Hartman; Nitin Gupta; Minchan
> Kim; Robert Jennings; Jenifer Hopper; Mel Gorman; Johannes Weiner; Rik van Riel; Larry Woodman;
> Benjamin Herrenschmidt; Joe Perches; Joonsoo Kim; Cody P Schafer; Hugh Dickens; Paul Mackerras; linux-
> mm@...ck.org; linux-kernel@...r.kernel.org; devel@...verdev.osuosl.org
> Subject: Re: [PATCHv11 3/4] zswap: add to mm/
> 
> On 05/15/2013 01:09 PM, Seth Jennings wrote:
> > On Wed, May 15, 2013 at 02:55:06PM -0400, Konrad Rzeszutek Wilk wrote:
> >>> Sorry, but I don't think that's appropriate for a patch in the MM subsystem.
> >>
> >> Perhaps a compromise can be reached where this code is merged as a driver
> >> not a core mm component. There is a high bar to be in the MM - it has to
> >> work with many many different configurations.
> >>
> >> And drivers don't have such a high bar. They just need to work on a specific
> >> issue and that is it. If zswap ended up in say, drivers/mm that would make
> >> it more palpable I think.
> 
> The issue is not whether it is a loadable module or a driver.  Nobody
> here is stupid enough to say, "hey, now it's a driver/module, all of the
> complex VM interactions are finally fixed!"
> 
> If folks don't want this in their system, there's a way to turn it off,
> today, with the sysfs tunables.  We don't need _another_ way to turn it
> off at runtime (unloading the module/driver).

The issue is we KNOW the complex VM interactions are NOT fixed
and there has been very very little breadth testing (i.e.
across a wide range of workloads, and any attempts to show
how much harm can come from enabling it.)

That's (at least borderline) acceptable in a driver that can
be unloaded, but not in MM code IMHO.
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