Received-SPF: pass (google.com: domain of linux-kernel+bounces-35530-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Message-ID: <90a9b1e5-9203-4baf-a69a-69b4f9ad73d5@arm.com>
Date: Tue, 23 Jan 2024 15:22:00 +0000
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and
 arm64
Content-Language: en-GB
To: Matthew Wilcox <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org,
 linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
 Russell King <linux@armlinux.org.uk>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Dinh Nguyen <dinguyen@kernel.org>, Michael Ellerman <mpe@ellerman.id.au>,
 Nicholas Piggin <npiggin@gmail.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt
 <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>,
 Alexander Gordeev <agordeev@linux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
 Heiko Carstens <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>,
 Christian Borntraeger <borntraeger@linux.ibm.com>,
 Sven Schnelle <svens@linux.ibm.com>, "David S. Miller"
 <davem@davemloft.net>, linux-arm-kernel@lists.infradead.org,
 linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
 linux-s390@vger.kernel.org, sparclinux@vger.kernel.org
References: <20240122194200.381241-1-david@redhat.com>
 <20240122194200.381241-2-david@redhat.com>
 <fdaeb9a5-d890-499a-92c8-d171df43ad01@arm.com>
 <Za_U0kYHGFkJdSss@casper.infradead.org>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <Za_U0kYHGFkJdSss@casper.infradead.org>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

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.