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bh=Wi8JCvfDMPalrSXsQQq726qxz61LBuEE0WYkMWBem1I=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=MdieU0FkgBD9jO+oaHHb+PAU9kNPVL8ASMuIuRPu9Bb+9eQH/ca8vRrM3DvNmi2wv P0JcOZauSNMXB9JRYOb4jbHCvFXAWy9LbhupAt+49YAEvtdX52igbRblbRKxDC6vHv muZXbhgWDd0yQKEsjKHTxULFBhdUdGv5Zxyp0Tsc= Date: Tue, 27 Oct 2026 16:49:45 +0000 From: Catalin Marinas To: "David Hildenbrand (Arm)" Cc: Zhao Gongyi , will@kernel.org, Ryan Roberts , dev.jain@arm.com, baolin.wang@linux.alibaba.com, lance.yang@linux.dev, mark.rutland@arm.com, akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] arm64/mm: restrict transparent contpte mappings to valid PTEs Message-ID: References: <20261013093600.54995-1-zhaogongyi@bytedance.com> <3b57c214-6a43-4fc7-a531-fb5eac196465@bytedance.com> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: C69094000B X-Stat-Signature: g65za4h7zzpa3wj5qoks551azetogry3 X-Rspam-User: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-HE-Tag: 1793119795-898555 X-HE-Meta: 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 mheniD+H XpgXKcPtyE5XvvkQidzkRM2k01wqvehi+Zw10bUQRGRLoh94U9ImBNGCqwBnqJJorC/G4gtHpm665jDsf0VK502esW5UKykI8cakLZfsCxlAZmp+27UnwEK39ZFxwgzbFVdjyi6MvoK+LXzWxJBNobzqV07Kb4iVoNpcukKoruYylCT1ZXMs3cicMDYfVRDsIToMiGLbAlZ+9ffnKTJECMyBHC3qzgyxEqb49ArCfiPsYb5FKAHEdrzMaIac9ILRjGBdGUKvuRIcxR2iEA1BgRyQu+PkMHUHsa5d9pmu6Le36hIIlOTZQHxOhN/dGiYaPHvb2ZQI8RmzClaa9ryTqfY0ESPM9OfaXH5bV Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Mon, Oct 26, 2026 at 11:33:31PM +0100, David Hildenbrand wrote: > On 10/26/26 13:30, Catalin Marinas wrote: > > On Sun, Oct 25, 2026 at 12:00:33AM +0800, Zhao Gongyi wrote: > >> On 10/24/26 4:55 AM, David Hildenbrand (Arm) wrote: > >>> If we have the spare PTE bit, why shouldn't we just use it? > >>> > >>> It will make batching over PTEs faster. And we care about that for present PTEs. > >>> > >>> So I don't quite see why the stable fix couldn't just do it right away? Do you > >>> have a prototype that shows that it would require significant code changes? > >> > >> I tried a small prototype that keeps PTE_CONT on present-invalid PTEs. > >> It leaves contpte_set_ptes() unchanged and makes contpte_try_unfold() > >> recognize pte_present(pte) && pte_cont(pte), instead of checking only > >> pte_valid_cont(pte). > > > > I think it's worth doing but currently not sure it buys us much unless > > we also change pte_batch_hint() to cover such entries. However, if we do > > this, we need to audit the other pte_valid_cont() callers (e.g. > > ptep_get() needs updating as well). > > > > So, if we aren't going to change pte_batch_hint() as a backport fix (I > > don't think we should, it's an optimisation), I'd rather take v2 now and > > continue with a proper pte_present_cont() etc. patchset on top. > > Your call, the version actually looks quite clean: > > https://lore.kernel.org/r/52e7e1f6-3535-474d-9b46-6bb62f96eefc@bytedance.com > > But yeah, audit needed (e.g., no need to flush when shattering such entries that > are not valid) Yes, the fix looked clean but I didn't particularly like the asymmetry between try_fold and try_unfold w.r.t. pte_present(). Well, that's more of an OCD ;). Going forward I think we should move towards pte_present_cont() but together with changing pte_batch_hint(). -- Catalin