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

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Date:   Thu, 9 Nov 2017 13:33:09 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     Jaewon Kim <jaewon31.kim@...il.com>
Cc:     Jaewon Kim <jaewon31.kim@...sung.com>,
        Andrew Morton <akpm@...ux-foundation.org>, mhocko@...e.com,
        vbabka@...e.cz, minchan@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: page_ext: allocate page extension though first PFN
 is invalid

On Wed, Nov 08, 2017 at 10:33:51PM +0900, Jaewon Kim wrote:
> 2017-11-08 16:52 GMT+09:00 Joonsoo Kim <iamjoonsoo.kim@....com>:
> > 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.
> Thank you for your comment.
> 
> AFAIK physical memory address depends on HW SoC.
> Sometimes a SoC remains few GB address region hole between few GB DRAM
> and other few GB DRAM
> such as 2GB under 4GB address and 2GB beyond 4GB address and holes between them.
> If SoC designs so big hole between actual mapping, I thought too much
> time will be spent on just checking all the PFNs.

I don't think that it is painful because it is done just once at
initialization step. However, if you worry about it, we can use
pfn_present() to skip the whole section at once. !pfn_present()
guarantees that there is no valid pfn in the section. If pfn_present()
returns true, we need to search the whole pages in the section in
order to find valid pfn.

And, I think that we don't need to change online_page_ext(). AFAIK,
hotplug always adds section aligned memory so pfn_present() check
should be enough.

Thanks.