linux-kernel - Re: [PATCH] mm: page_ext: allocate page extension though first PFN is invalid

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Date:   Wed, 8 Nov 2017 16:52:42 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     Jaewon Kim <jaewon31.kim@...sung.com>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com, vbabka@...e.cz,
        minchan@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jaewon31.kim@...il.com
Subject: Re: [PATCH] mm: page_ext: allocate page extension though first PFN
 is invalid

On Tue, Nov 07, 2017 at 06:44:47PM +0900, Jaewon Kim wrote:
> online_page_ext and page_ext_init allocate page_ext for each section, but
> they do not allocate if the first PFN is !pfn_present(pfn) or
> !pfn_valid(pfn).
> 
> Though the first page is not valid, page_ext could be useful for other
> pages in the section. But checking all PFNs in a section may be time
> consuming job. Let's check each (section count / 16) PFN, then prepare
> page_ext if any PFN is present or valid.

I guess that this kind of section is not so many. And, this is for
debugging so completeness would be important. It's better to check
all pfn in the section.

Thanks.