linux-kernel - Re: [PATCHv13 2/4] zbud: add to mm/

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Message-ID: <20130605135508.GA25375@cerebellum>
Date:	Wed, 5 Jun 2013 08:55:08 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Bob Liu <bob.liu@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...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@...1.net>, 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: [PATCHv13 2/4] zbud: add to mm/

On Wed, Jun 05, 2013 at 02:43:28PM +0800, Bob Liu wrote:
> Hi Seth,
> 
> On 06/04/2013 04:33 AM, Seth Jennings wrote:
> > +	/* Couldn't find unbuddied zbud page, create new one */
> 
> How about moving zswap_is_full() to here.
> 
> if (zswap_is_full()) {
> 	/* Don't alloc any new page, try to reclaim and direct use the
> reclaimed page instead */

Yes, this is at the top of the list for improvements.

I have already started on this work and it isn't quite as simple as it seems.
The difficulty rises from the fact that, for now, zswap uses per-cpu
compression buffers which require preemption to be disabled. This prevents the
calling zbud_reclaim_page() in zbud_alloc() because the eviction handler for
the user may do something that can wait; an allocation with GFP_WAIT for
example.

So it's going to take some massaging in the zswap layer to get that to work.

It's very doable.  Just not in this patchset without causing a lot of code
thrash.

> }
> 
> > +	spin_unlock(&pool->lock);
> > +	page = alloc_page(gfp);
> > +	if (!page)
> > +		return -ENOMEM;
> > +	spin_lock(&pool->lock);
> > +	pool->pages_nr++;
> > +	zhdr = init_zbud_page(page);
> > +	bud = FIRST;
<snip>
> 
> It looks good for me except two things.
> One is about what the performance might be after the zswap pool is full.
> The other is still about the 20% limit of zswap pool size.

Yep, working on both of them.

Seth

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