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Date:   Thu, 24 May 2018 16:56:11 +0800
From:   Dave Young <dyoung@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org,
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Subject: Re: [PATCH v1 00/10] mm: online/offline 4MB chunks controlled by
 device driver
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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