linux-kernel - Re: [PATCH v2 0/6] Reduce memory waste by page extension user

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Message-ID: <20160816101202.GD17417@dhcp22.suse.cz>
Date:	Tue, 16 Aug 2016 12:12:02 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	js1304@...il.com
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Minchan Kim <minchan@...nel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 0/6] Reduce memory waste by page extension user

On Tue 16-08-16 11:53:00, Michal Hocko wrote:
> On Tue 16-08-16 11:51:13, Joonsoo Kim wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> > v2:
> > Fix rebase mistake (per Vlastimil)
> > Rename some variable/function to prevent confusion (per Vlastimil)
> > Fix header dependency (per Sergey)
> > 
> > This patchset tries to reduce memory waste by page extension user.
> > 
> > First case is architecture supported debug_pagealloc. It doesn't
> > requires additional memory if guard page isn't used. 8 bytes per
> > page will be saved in this case.
> > 
> > Second case is related to page owner feature. Until now, if page_ext
> > users want to use it's own fields on page_ext, fields should be
> > defined in struct page_ext by hard-coding. It has a following problem.
> > 
> > struct page_ext {
> >  #ifdef CONFIG_A
> > 	int a;
> >  #endif
> >  #ifdef CONFIG_B
> > 	int b;
> >  #endif
> > };
> > 
> > Assume that kernel is built with both CONFIG_A and CONFIG_B.
> > Even if we enable feature A and doesn't enable feature B at runtime,
> > each entry of struct page_ext takes two int rather than one int.
> > It's undesirable waste so this patch tries to reduce it. By this patchset,
> > we can save 20 bytes per page dedicated for page owner feature
> > in some configurations.
> 
> FWIW I like this. I have only glanced over those patches so I do not
> feel comfortable to give my a-b but the approach is sensible and the
> memory savings are really attractive. Page owner is a really great
> debugging feauture so enabling it makes a lot of sense on production
> servers where the memory wasting is a no-go.

OK, I have missed that page_ext is only allocated if there is at least
one feature which requires it enabled. So normally there shouldn't be
too much of wasted memory. Anyway allocating per feature makes a lot of
sense regardless.
-- 
Michal Hocko
SUSE Labs