linux-kernel - Re: [PATCH v1 00/10] mm: online/offline 4MB chunks controlled by device driver

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Date:   Thu, 24 May 2018 16:56:11 +0800
From:   Dave Young <dyoung@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Balbir Singh <bsingharora@...il.com>,
        Baoquan He <bhe@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Huang Ying <ying.huang@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>, Jan Kara <jack@...e.cz>,
        Jérôme Glisse <jglisse@...hat.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Juergen Gross <jgross@...e.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Mel Gorman <mgorman@...e.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Miles Chen <miles.chen@...iatek.com>,
        Oscar Salvador <osalvador@...hadventures.net>,
        Paul Mackerras <paulus@...ba.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Rashmica Gupta <rashmica.g@...il.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v1 00/10] mm: online/offline 4MB chunks controlled by
 device driver

Hi,

[snip]
> > 
> >> For kdump and onlining/offlining code, we
> >> have to mark pages as offline before a new segment is visible to the system
> >> (e.g. as these pages might not be backed by real memory in the hypervisor).
> > 
> > Please expand on the kdump part. That is really confusing because
> > hotplug should simply not depend on kdump at all. Moreover why don't you
> > simply mark those pages reserved and pull them out from the page
> > allocator?
> 
> 1. "hotplug should simply not depend on kdump at all"
> 
> In theory yes. In the current state we already have to trigger kdump to
> reload whenever we add/remove a memory block.
> 
> 
> 2. kdump part
> 
> Whenever we offline a page and tell the hypervisor about it ("unplug"),
> we should not assume that we can read that page again. Now, if dumping
> tools assume they can read all memory that is offline, we are in trouble.
> 
> It is the same thing as we already have with Pg_hwpoison. Just a
> different meaning - "don't touch this page, it is offline" compared to
> "don't touch this page, hw is broken".

Does that means in case an offline no kdump reload as mentioned in 1)?

If we have the offline event and reload kdump, I assume the memory state
is refreshed so kdump will not read the memory offlined, am I missing
something?

> 
> Balloon drivers solve this problem by always allowing to read unplugged
> memory. In virtio-mem, this cannot and should even not be guaranteed.
> 

Hmm, that sounds a bug..

> And what we have to do to make this work is actually pretty simple: Just
> like Pg_hwpoison, track per page if it is online and provide this
> information to kdump.
> 
> 

Thanks
Dave