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

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Message-ID: <20160816095300.GC17417@dhcp22.suse.cz>
Date:	Tue, 16 Aug 2016 11:53:01 +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 11:51:13, 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.

Thanks!
-- 
Michal Hocko
SUSE Labs