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

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Message-ID: <20191022092454.GI9379@dhcp22.suse.cz>
Date:   Tue, 22 Oct 2019 11:24:54 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
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 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
can very well test for ZONE_DEVICE as well. PageReserved -> ZONE_DEVICE
is a terrible assumption.

> I am reworking these patches right now and audit the whole kernel for
> PageReserved() checks that might affect ZONE_DEVICE. I'll send the
> collection of patches as RFC.

Thanks a lot!
-- 
Michal Hocko
SUSE Labs