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

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Date:   Thu, 26 Jul 2018 10:27:23 +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 Wed 25-07-18 16:20:41, David Hildenbrand wrote:
> On 25.07.2018 15:51, Michal Hocko wrote:
> > On Tue 24-07-18 16:13:09, David Hildenbrand wrote:
> > [...]
> >> So I see right now:
> >>
> >> - Pg_reserved + e.g. new page type (or some other unique identifier in
> >>   combination with Pg_reserved)
> >>  -> Avoid reads of pages we know are offline
> >> - extend is_ram_page()
> >>  -> Fake zero memory for pages we know are offline
> >>
> >> Or even both (avoid reading and don't crash the kernel if it is being done).
> > 
> > I really fail to see how that can work without kernel being aware of
> > PageOffline. What will/should happen if you run an old kdump tool on a
> > kernel with this partially offline memory?
> > 
> 
> New kernel with old dump tool:
> 
> a) we have not fixed up is_ram_page()
> 
> -> crash, as we access memory we shouldn't

this is not acceptable, right? You do not want to crash your crash
kernel ;)

> b) we have fixed up is_ram_page()
> 
> -> We have a callback to check for applicable memory in the hypervisor
> whether the parts are accessible / online or not accessible / offline.
> (e.g. via a device driver that controls a certain memory region)
> 
> -> Don't read, but fake a page full of 0
> 
> 
> So instead of the kernel being aware of it, it asks via is_ram_page()
> the hypervisor.

I am still confused why do we even care about hypervisor. What if
somebody wants to have partial memory hotplug on native OS?
 
> I don't think a) is a problem. AFAICS, we have to update makedumpfile
> for every new kernel. We can perform changes and update makedumpfile
> to be compatible with new dump tools.

Not really. You simply do not crash the kernel just because you are
trying to dump the already crashed kernel.

> E.g. remember SECTION_IS_ONLINE you introduced ? It broke dump
> tools and required

But has it crashed the kernel when reading the dump? If yes then the
whole dumping is fragile as hell...
-- 
Michal Hocko
SUSE Labs