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Subject: Re: [PATCH 0/4] arch, mm: improve robustness of direct map
 manipulation
To: Mike Rapoport <rppt@kernel.org>, Andrew Morton <akpm@linux-foundation.org>
References: <20201025101555.3057-1-rppt@kernel.org>
From: David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
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Date: Thu, 29 Oct 2020 09:15:47 +0100
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Cc: Peter Zijlstra <peterz@infradead.org>,
 Dave Hansen <dave.hansen@linux.intel.com>, linux-mm@kvack.org,
 Paul Mackerras <paulus@samba.org>, Pavel Machek <pavel@ucw.cz>,
 "H. Peter Anvin" <hpa@zytor.com>, sparclinux@vger.kernel.org,
 Christoph Lameter <cl@linux.com>, Will Deacon <will@kernel.org>,
 linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, x86@kernel.org,
 Mike Rapoport <rppt@linux.ibm.com>,
 Christian Borntraeger <borntraeger@de.ibm.com>, Ingo Molnar <mingo@redhat.com>,
 Catalin Marinas <catalin.marinas@arm.com>, Len Brown <len.brown@intel.com>,
 Albert Ou <aou@eecs.berkeley.edu>, Vasily Gorbik <gor@linux.ibm.com>,
 linux-pm@vger.kernel.org, Heiko Carstens <hca@linux.ibm.com>,
 David Rientjes <rientjes@google.com>, Borislav Petkov <bp@alien8.de>,
 Andy Lutomirski <luto@kernel.org>, Paul Walmsley <paul.walmsley@sifive.com>,
 "Kirill A. Shutemov" <kirill@shutemov.name>,
 Thomas Gleixner <tglx@linutronix.de>, linux-arm-kernel@lists.infradead.org,
 "Rafael J. Wysocki" <rjw@rjwysocki.net>, linux-kernel@vger.kernel.org,
 Pekka Enberg <penberg@kernel.org>, Palmer Dabbelt <palmer@dabbelt.com>,
 Joonsoo Kim <iamjoonsoo.kim@lge.com>, "Edgecombe,
 Rick P" <rick.p.edgecombe@intel.com>, linuxppc-dev@lists.ozlabs.org,
 "David S. Miller" <davem@davemloft.net>
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On 25.10.20 11:15, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> Hi,
> 
> During recent discussion about KVM protected memory, David raised a concern
> about usage of __kernel_map_pages() outside of DEBUG_PAGEALLOC scope [1].
> 
> Indeed, for architectures that define CONFIG_ARCH_HAS_SET_DIRECT_MAP it is
> possible that __kernel_map_pages() would fail, but since this function is
> void, the failure will go unnoticed.
> 
> Moreover, there's lack of consistency of __kernel_map_pages() semantics
> across architectures as some guard this function with
> #ifdef DEBUG_PAGEALLOC, some refuse to update the direct map if page
> allocation debugging is disabled at run time and some allow modifying the
> direct map regardless of DEBUG_PAGEALLOC settings.
> 
> This set straightens this out by restoring dependency of
> __kernel_map_pages() on DEBUG_PAGEALLOC and updating the call sites
> accordingly.
> 

So, I was primarily wondering if we really have to touch direct mappings 
in hibernation code, or if we can avoid doing that. I was wondering if 
we cannot simply do something like kmap() when trying to access a 
!mapped page. Similar to reading old-os memory after kexec when in 
kdump. Just a thought.

-- 
Thanks,

David / dhildenb