linux-kernel - Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE

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Date: Tue, 23 Jan 2024 15:22:00 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Matthew Wilcox <willy@...radead.org>
Cc: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Russell King <linux@...linux.org.uk>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Dinh Nguyen <dinguyen@...nel.org>, Michael Ellerman <mpe@...erman.id.au>,
 Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller"
 <davem@...emloft.net>, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-s390@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and
 arm64

On 23/01/2024 15:01, Matthew Wilcox wrote:
> On Tue, Jan 23, 2024 at 10:34:21AM +0000, Ryan Roberts wrote:
>>> +#define PFN_PTE_SHIFT		PAGE_SHIFT
>>
>> I think this is buggy. And so is the arm64 implementation of set_ptes(). It
>> works fine for 48-bit output address, but for 52-bit OAs, the high bits are not
>> kept contigously, so if you happen to be setting a mapping for which the
>> physical memory block straddles bit 48, this won't work.
> 
> I'd like to see the folio allocation that can straddle bit 48 ...
> 
> agreed, it's not workable _in general_, but specifically for a memory
> allocation from a power-of-two allocator, you'd have to do a 49-bit
> allocation (half a petabyte) to care.

Hmm good point. So its a hypothetical bug, not an actual bug. Personally I'm
still inclined to "fix" it. Although its going to cost a few more instructions.
Shout if you disagree.