linux-kernel - Re: [PATCH v2 2/4] arm64: hugetlb: Fix huge_ptep_get_and

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Message-ID: <20250225221812.GA23870@willie-the-truck>
Date: Tue, 25 Feb 2025 22:18:13 +0000
From: Will Deacon <will@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Huacai Chen <chenhuacai@...nel.org>,
	WANG Xuerui <kernel@...0n.name>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	"James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
	Helge Deller <deller@....de>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Naveen N Rao <naveen@...nel.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Andreas Larsson <andreas@...sler.com>,
	Arnd Bergmann <arnd@...db.de>, Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	David Hildenbrand <david@...hat.com>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Dev Jain <dev.jain@....com>, Kevin Brodsky <kevin.brodsky@....com>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/4] arm64: hugetlb: Fix huge_ptep_get_and_clear() for
 non-present ptes

On Mon, Feb 24, 2025 at 12:11:19PM +0000, Ryan Roberts wrote:
> On 21/02/2025 15:31, Will Deacon wrote:
> > On Mon, Feb 17, 2025 at 02:04:15PM +0000, Ryan Roberts wrote:
> >> +	pte = __ptep_get_and_clear(mm, addr, ptep);
> >> +	present = pte_present(pte);
> >> +	while (--ncontig) {
> >> +		ptep++;
> >> +		addr += pgsize;
> >> +		tmp_pte = __ptep_get_and_clear(mm, addr, ptep);
> >> +		if (present) {
> >> +			if (pte_dirty(tmp_pte))
> >> +				pte = pte_mkdirty(pte);
> >> +			if (pte_young(tmp_pte))
> >> +				pte = pte_mkyoung(pte);
> >> +		}
> >>  	}
> > 
> > nit: With the loop now structured like this, we really can't handle
> > num_contig_ptes() returning 0 if it gets an unknown size. Granted, that
> > really shouldn't happen, but perhaps it would be better to add a 'default'
> > case with a WARN() to num_contig_ptes() and then add an early return here?
> 
> Looking at other users of num_contig_ptes() it looks like huge_ptep_get()
> already assumes at least 1 pte (it calls __ptep_get() before calling
> num_contig_ptes()) and set_huge_pte_at() assumes 1 pte for the "present and
> non-contig" case. So num_contig_ptes() returning 0 is already not really
> consumed consistently.
> 
> How about we change the default num_contig_ptes() return value to 1 and add a
> warning if size is invalid:

Fine by me!

I assume you'll fold that in and send a new version, along with the typo
fixes?

Cheers,

Will