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Date:   Mon, 1 Jun 2020 14:42:46 +0300
From:   Mike Rapoport <rppt@linux.ibm.com>
To:     Steve Wahl <steve.wahl@hpe.com>
Cc:     Baoquan He <bhe@redhat.com>, David Hildenbrand <david@redhat.com>,
        mgorman@suse.de, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, cai@lca.pw, mhocko@kernel.org
Subject: Re: [PATCH] mm/compaction: Fix the incorrect hole in
 fast_isolate_freepages()
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On Thu, May 28, 2020 at 10:15:10AM -0500, Steve Wahl wrote:
> On Thu, May 28, 2020 at 05:07:31PM +0800, Baoquan He wrote:
> > On 05/26/20 at 01:49pm, David Hildenbrand wrote:
> > > On 26.05.20 13:32, Mike Rapoport wrote:
> > > > Hello Baoquan,
> > > > 
> > > > I do not object to your original fix with careful check for pfn validity.
> > > > 
> > > > But I still think that the memory reserved by the firmware is still
> > > > memory and it should be added to memblock.memory. This way the memory
> > > 
> > > If it's really memory that could be read/written, I think I agree.
> > 
> > I would say some of them may not be allowed to be read/written, if I
> > understand it correctly. I roughly went through the x86 init code, there
> > are some places where mem region is marked as E820_TYPE_RESERVED so that
> > they are not touched after initialization. E.g:
> > 
> > 1) pfn 0
> > In trim_bios_range(), we set the pfn 0 as E820_TYPE_RESERVED. You can
> > see the code comment, this is a BIOS owned area, but not kernel RAM.
> > 
> > 2)GART reserved region
> > In early_gart_iommu_check(), GART IOMMU firmware will reserve a region
> > in an area, firmware designer won't map system RAM into that area.
> > 
> > And also intel_graphics_stolen(), arch_rmrr_sanity_check(), these
> > regions are not system RAM backed area, reading from or writting into
> > these area may cause error.
> > 
> > Futhermore, there's a KASLR bug found by HPE, its triggering and root
> > cause are written into below commit log. You can see that accessing to
> > firmware reserved region caused BIOS to halt system when cpu doing
> > speculative.
> > 
> > commit 2aa85f246c181b1fa89f27e8e20c5636426be624
> > Author: Steve Wahl <steve.wahl@hpe.com>
> > Date:   Tue Sep 24 16:03:55 2019 -0500
> > 
> >     x86/boot/64: Make level2_kernel_pgt pages invalid outside kernel area
> > 
> >     Our hardware (UV aka Superdome Flex) has address ranges marked
> >     reserved by the BIOS. Access to these ranges is caught as an error,
> >     causing the BIOS to halt the system.
> 
> Thank you for CC'ing me.  Coming into the middle of the conversation,
> I am trying to understand the implications of what's being discussed
> here, but not being very successful at it.
> 
> For the HPE UV hardware, the addresses listed in the reserved range
> must never be accessed, or (as we discovered) even be reachable by an
> active page table entry.  Any active page table entry covering the
> region allows the processor to issue speculative accesses to the
> region, resulting in the BIOS halt mentioned.
> 
> I'm not sure if the discussion above about adding the region to
> memblock.memory would result in an active mapping of the region or
> not.  If it did, we'd have problems.

The discussion is whether regions marked as E820_RESERVED should be
considered as RAM or not.

For the hardware that cannot tolerate acceses to these areas like HPE
UV, it should not :)

I still think that keeping them only in memblock.reserved creates more
problems than it solves, but simply adding E820_RESERVED areas to
memblock.memory just won't work.

I'll try to come up with something better Really Soon (c).

> Thanks,
> 
> Steve Wahl, HPE

-- 
Sincerely yours,
Mike.