linux-kernel - Re: [PATCH] switch free memory back to MIGRATE

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Date:	Mon, 13 Jul 2009 14:38:57 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"mel@....ul.ie" <mel@....ul.ie>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] switch free memory back to MIGRATE_MOVABLE

On Mon, 13 Jul 2009 11:18:01 +0800
Shaohua Li <shaohua.li@...el.com> wrote:

> On Mon, Jul 13, 2009 at 11:08:14AM +0800, KOSAKI Motohiro wrote:
> > > On Mon, Jul 13, 2009 at 10:58:03AM +0800, KAMEZAWA Hiroyuki wrote:
> > > > On Mon, 13 Jul 2009 11:47:46 +0900 (JST)
> > > > KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> > > > 
> > > > > > When page is back to buddy and its order is bigger than pageblock_order, we can
> > > > > > switch its type to MIGRATE_MOVABLE. This can reduce fragmentation. The patch
> > > > > > has obvious effect when read a block device and then drop caches.
> > > > > > 
> > > > > > Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> > > > > 
> > > > > This patch change hot path, but there is no performance mesurement description.
> > > > > Also, I don't like modification buddy core for only drop caches.
> > > > > 
> > > > Li, does this patch imply fallback of migration type doesn't work well ?
> > > > What is the bad case ?
> > > The page is initialized as migrate_movable, and then switch to reclaimable or
> > > something else when fallback occurs, but its type remains even the page gets
> > > freed. When the page gets freed, its type actually can be switch back to movable,
> > > this is what the patch does.
> > 
> > This answer is not actual answer.
> > Why do you think __rmqueue_fallback() doesn't works well? Do you have
> > any test-case or found a bug by review?
> I never said __rmqueue_fallback() doesn't work well. The page is already freed, switching
> back the pageblock to movable might make next page allocation (non-movable) skip this
> pageblock. So this could potentially reduce fragmentation and improve memory offline.
> But your guys are right, I have no number if this will impact performance.
> 
If this is for memory offlining, plz mention that at first ;)
IIUC, if this can be a problem, fixing memory offline itself is better. No ?
At implementing memory unplug, I had no problems because I assumes ZONE_MOVABLE.
But ok, I welcome enhances to memory unplug.

If this part is bad for you.
4714         /*
4715          * In future, more migrate types will be able to be isolation target.
4716          */
4717         if (get_pageblock_migratetype(page) != MIGRATE_MOVABLE)
4718                 goto out;

plz fix this to do more precise work for zid != ZONE_MOVABLE zones.
As I wrote in comments. My codes assumes ZONE_MOVABLE in many parts because I want
100%-success memory offline. 

Thanks,
-Kame

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