linux-kernel - Re: [PATCH v9 08/12] mm: zero reserved and unavailable struct pages

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Message-ID: <691dba28-718c-e9a9-d006-88505eb5cd7e@oracle.com>
Date:   Tue, 3 Oct 2017 11:29:16 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, kasan-dev@...glegroups.com, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, davem@...emloft.net,
        willy@...radead.org, ard.biesheuvel@...aro.org,
        mark.rutland@....com, will.deacon@....com, catalin.marinas@....com,
        sam@...nborg.org, mgorman@...hsingularity.net,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        bob.picco@...cle.com
Subject: Re: [PATCH v9 08/12] mm: zero reserved and unavailable struct pages

On 10/03/2017 09:18 AM, Michal Hocko wrote:
> On Wed 20-09-17 16:17:10, Pavel Tatashin wrote:
>> Some memory is reserved but unavailable: not present in memblock.memory
>> (because not backed by physical pages), but present in memblock.reserved.
>> Such memory has backing struct pages, but they are not initialized by going
>> through __init_single_page().
> 
> Could you be more specific where is such a memory reserved?
> 

I know of one example: trim_low_memory_range() unconditionally reserves 
from pfn 0, but e820__memblock_setup() might provide the exiting memory 
from pfn 1 (i.e. KVM).

But, there could be more based on this comment from linux/page-flags.h:

  19  * PG_reserved is set for special pages, which can never be swapped 
out. Some
  20  * of them might not even exist (eg empty_bad_page)...

Pasha