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

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Date: Tue, 23 Jan 2024 15:01:38 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Ryan Roberts <ryan.roberts@....com>
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 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.