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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z7hNgSBw6lfwwcch@arm.com>
Date: Fri, 21 Feb 2025 09:55:13 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: Ryan Roberts <ryan.roberts@....com>, Will Deacon <will@...nel.org>,
	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>, 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 Thu, Feb 20, 2025 at 12:07:35PM +0530, Anshuman Khandual wrote:
> On 2/19/25 14:28, Ryan Roberts wrote:
> > On 19/02/2025 08:45, Anshuman Khandual wrote:
> >> On 2/17/25 19:34, Ryan Roberts wrote:
> >>> +	while (--ncontig) {
> >>
> >> Should this be converted into a for loop instead just to be in sync with other
> >> similar iterators in this file.
> >>
> >> for (i = 1; i < ncontig; i++, addr += pgsize, ptep++)
> >> {
> >> 	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);
> >> 	}
> >> }
> > 
> > I think the way you have written this it's incorrect. Let's say we have 16 ptes
> > in the block. We want to iterate over the last 15 of them (we have already read
> > pte 0). But you're iterating over the first 15 because you don't increment addr
> > and ptep until after you've been around the loop the first time. So we would
> > need to explicitly increment those 2 before entering the loop. But that is only
> > neccessary if ncontig > 1. Personally I think my approach is neater...
> 
> Thinking about this again. Just wondering should not a pte_present()
> check on each entries being cleared along with (ncontig > 1) in this
> existing loop before transferring over the dirty and accessed bits -
> also work as intended with less code churn ?

Shouldn't all the ptes in a contig block be either all present or all
not-present? Is there any point in checking each individually?

-- 
Catalin