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

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Message-ID: <bc5af769-6d3a-4003-81c2-f5fe5cf8550c@arm.com>
Date: Wed, 26 Feb 2025 08:09:32 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Will Deacon <will@...nel.org>
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 25/02/2025 22:18, Will Deacon wrote:
> 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?

Yep, I'll aim to post this today. I have a few review comments for s390 to add
in too.

> 
> Cheers,
> 
> Will