linux-kernel - Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps

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Date:   Thu, 26 Jul 2018 10:30:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>, Dave Young <dyoung@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Huang Ying <ying.huang@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Marc-André Lureau 
        <marcandre.lureau@...hat.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Miles Chen <miles.chen@...iatek.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Petr Tesarik <ptesarik@...e.cz>
Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps

On Thu 26-07-18 10:22:41, David Hildenbrand wrote:
> On 24.07.2018 09:22, Michal Hocko wrote:
> > On Mon 23-07-18 19:12:58, David Hildenbrand wrote:
> >> On 23.07.2018 13:45, Vlastimil Babka wrote:
> >>> On 07/20/2018 02:34 PM, David Hildenbrand wrote:
> >>>> Dumping tools (like makedumpfile) right now don't exclude reserved pages.
> >>>> So reserved pages might be access by dump tools although nobody except
> >>>> the owner should touch them.
> >>>
> >>> Are you sure about that? Or maybe I understand wrong. Maybe it changed
> >>> recently, but IIRC pages that are backing memmap (struct pages) are also
> >>> PG_reserved. And you definitely do want those in the dump.
> >>
> >> I proposed a new flag/value to mask pages that are logically offline but
> >> Michal wanted me to go into this direction.
> >>
> >> While we can special case struct pages in dump tools ("we have to
> >> read/interpret them either way, so we can also dump them"), it smells
> >> like my original attempt was cleaner. Michal?
> > 
> > But we do not have many page flags spare and even if we have one or two
> > this doesn't look like the use for them. So I still think we should try
> > the PageReserved way.
> > 
> 
> So as a summary, the only real approach that would be acceptable is
> using PageReserved + some other identifier to mark pages as "logically
> offline".
> 
> I wonder what identifier could be used, as this has to be consistent for
> all reserved pages (to avoid false positives).
> 
> Using other pageflags in combination might be possible, but then we have
> to make assumptions about all users of PageReserved right now.
> 
> As far as I can see (and as has been discussed), page_type could be
> used. If we don't want to consume a new bit, we could overload/reuse the
> "PG_balloon" bit.
> 
> 
> E.g. "PG_balloon" set -> exclude page from dump

Does each user of PG_balloon check for PG_reserved? If this is the case
then yes this would be OK.
-- 
Michal Hocko
SUSE Labs