linux-kernel - Re: [PATCH v2 0/2] mm: Memory offlining + page isolation cleanups

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Message-ID: <8f5a1d58-6b1a-eef3-b482-ff113425fa65@redhat.com>
Date:   Tue, 22 Oct 2019 11:27:18 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Pingfan Liu <kernelfans@...il.com>, Qian Cai <cai@....pw>,
        Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...il.com>
Subject: Re: [PATCH v2 0/2] mm: Memory offlining + page isolation cleanups

On 22.10.19 11:24, Michal Hocko wrote:
> On Tue 22-10-19 11:17:24, David Hildenbrand wrote:
>> On 22.10.19 11:14, Michal Hocko wrote:
>>> On Tue 22-10-19 10:32:11, David Hildenbrand wrote:
>>> [...]
>>>> E.g., arch/x86/kvm/mmu.c:kvm_is_mmio_pfn()
>>>
>>> Thanks for these references. I am not really familiar with kvm so I
>>> cannot really comment on the specific code but I am wondering why
>>> it simply doesn't check for ZONE_DEVICE explicitly? Also we do care
>>> about holes in RAM (from the early boot), those should be reserved
>>> already AFAIR. So we are left with hotplugged memory with holes and
>>> I am not really sure we should bother with this until there is a clear
>>> usecase in sight.
>>
>> Well, checking for ZONE_DEVICE is only possible if you have an initialized
>> memmap. And that is not guaranteed when you start mapping random stuff into
>> your guest via /dev/mem.
> 
> Yes, I can understand that part but checking PageReserved on an
> uninitialized memmap is pointless as well. So if you can test for it you

That's why I add pfn_to_online_page() :)

> can very well test for ZONE_DEVICE as well. PageReserved -> ZONE_DEVICE
> is a terrible assumption.
Indeed, it is. But there are more parts in the kernel that I'll be 
fixing. Stay tuned.

-- 

Thanks,

David / dhildenb