linux-kernel - Re: [RFC PATCH 2/4] mm: make consistent use of PG

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Date:	Thu, 5 Jul 2012 12:37:41 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Jiang Liu <liuj97@...il.com>
cc:	Jiang Liu <jiang.liu@...wei.com>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, Mel Gorman <mgorman@...e.de>,
	Yinghai Lu <yinghai@...nel.org>,
	Tony Luck <tony.luck@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Keping Chen <chenkeping@...wei.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/4] mm: make consistent use of PG_slab flag

On Fri, 6 Jul 2012, Jiang Liu wrote:

> On 07/05/2012 10:47 PM, Christoph Lameter wrote:
> > On Tue, 3 Jul 2012, Jiang Liu wrote:
> >
> >> PG_slabobject:	mark whether a (compound) page hosts SLUB/SLOB objects.
> >
> > Any subsystem may allocate a compound page to store metadata.
> >
> > The compound pages used by SLOB and SLUB are not managed in any way but
> > the calls to kfree and kmalloc are converted to calls to the page
> > allocator. There is no "management" by the slab allocators for these
> > cases and its inaccurate to say that these are SLUB/SLOB objects since the
> > allocators never deal with these objects.
> >
> Hi Chris,
> 	I think there's a little difference with SLUB and SLOB for compound page.
> For SLOB, it relies on the page allocator to allocate compound page to fulfill
> request bigger than one page. For SLUB, it relies on the page allocator if the
> request is bigger than two pages. So SLUB may allocate a 2-pages compound page
> to host SLUB managed objects.
> 	My proposal may be summarized as below:
> 	1) PG_slab flag marks a memory object is allocated from slab allocator.
> 	2) PG_slabobject marks a (compound) page hosts SLUB/SLOB managed objects.
> 	3) Only set PG_slab/PG_slabobject on the head page of compound pages.
> 	4) For SLAB, PG_slabobject is redundant and so not used.
>
> 	A summary of proposed usage of PG_slab(S) and PG_slabobject(O) with
> SLAB/SLUB/SLOB allocators as below:
> pagesize	SLAB			SLUB			SLOB
> 1page		S			S,O			S,O
> 2page		S			S,O			S
> >=4page		S			S			S

There is no point of recognizing such objects because those will be
kmalloc objects and they can only be freed in a subsystem specific way.
There is no standard way to even figure out which subsystem allocated
them. So for all practical purposes those are unrecoverable.


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