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

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Message-ID: <20171004085636.w2rnwf5xxhahzuy7@dhcp22.suse.cz>
Date:   Wed, 4 Oct 2017 10:56:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pasha Tatashin <pasha.tatashin@...cle.com>
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 Tue 03-10-17 11:29:16, Pasha Tatashin wrote:
> 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).

Then just initialize struct pages for that mapping rigth there where a
special API is used.

> 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)...

I have no idea wht empty_bad_page is but a quick grep shows that this is
never used. I might be wrong here but if somebody is reserving a memory
in a special way then we should handle the initialization right there.
E.g. create an API for special memblock reservations.
-- 
Michal Hocko
SUSE Labs