linux-kernel - Re: [PATCH 1/3] mm: consider zone which is not fully populated to have holes

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Date:   Wed, 19 Apr 2017 14:50:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Joonsoo Kim <js1304@...il.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Kani Toshimitsu <toshi.kani@....com>,
        slaoub@...il.com, Andi Kleen <ak@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] mm: consider zone which is not fully populated to
 have holes

On Wed 19-04-17 14:34:54, Vlastimil Babka wrote:
> On 04/19/2017 02:16 PM, Michal Hocko wrote:
> > On Wed 19-04-17 13:59:40, Vlastimil Babka wrote:
> >> On 04/18/2017 11:27 AM, Michal Hocko wrote:
> > [...]
> >>> I am not aware of any such user. PageReserved has always been about "the
> >>> core mm should touch these pages and modify their state" AFAIR.
> >>> But I believe that touching those holes just asks for problems so I
> >>> would rather have them covered.
> >>
> >> OK. I guess it's OK to use PageReserved of first pageblock page to
> >> determine if we can trust page_zone(), because the memory offline
> >> scenario should have sufficient granularity and not make holes inside
> >> pageblock?
> > 
> > Yes memblocks should be section size aligned and that is 128M resp. 2GB
> > on large machines. So we are talking about much larger than page block
> > granularity here.
> > 
> > Anyway, Joonsoo didn't like the the explicit PageReserved checks so I
> > have come with pfn_to_online_page which hides this implementation
> > detail. How do you like the following instead?
> 
> Yeah that's OK. The other two patches will be updated as well?

yes

> Ideally we would later convert this helper to use some special values
> for zone/node id (such as -1) instead of PageReserved to indicate an
> offline node, as we discussed.

I have considered zone_id to be -1 but there is just too much code which
uses the id to translate it to the struct zone * directly and that would
lead to subtle bugs. On the other hand zone_id == 0 is not optimal but
much safer from that POV. I will think about the safest way forward long
term but my intention was to have something reasonably good for starter.
-- 
Michal Hocko
SUSE Labs