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[2003:cb:c711:c600:c1d6:7158:f946:f083]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3749ef7e09esm1639187f8f.86.2024.08.29.08.10.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Aug 2024 08:10:16 -0700 (PDT) Message-ID: Date: Thu, 29 Aug 2024 17:10:15 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 06/19] mm/pagewalk: Check pfnmap for folio_walk_start() To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Gavin Shan , Catalin Marinas , x86@kernel.org, Ingo Molnar , Andrew Morton , Paolo Bonzini , Dave Hansen , Thomas Gleixner , Alistair Popple , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Sean Christopherson , Oscar Salvador , Jason Gunthorpe , Borislav Petkov , Zi Yan , Axel Rasmussen , Yan Zhao , Will Deacon , Kefeng Wang , Alex Williamson References: <20240826204353.2228736-1-peterx@redhat.com> <20240826204353.2228736-7-peterx@redhat.com> <9f9d7e96-b135-4830-b528-37418ae7bbfd@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 21C251C0032 X-Stat-Signature: 9gkwhdbpsx3po8x7mn7ewkha6qmmkngo X-Rspam-User: X-HE-Tag: 1724944221-501048 X-HE-Meta: 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 Y+vllpId kpoPrn1cXwJlr1Tox9L4017+elkJxQQhGXQqSjEZ10Ka2Dh7OosKbZKi5HrWt0XzLpIu7NiOZgi7eOS6iX47lq8lcoi0FAAszYmGjFcPLELZW7RQz4gs7Fz1GlsEMXFxKkaMUmrXCvVMkpd2Qng3ub7gQIkxFH+s1kwMFjkTg32thDK6PMqo6wdv3qHMIsTErOupAVTs1c/F0OOHjVmX4+uK/+iLxcKyy46OYJ6rDtIUkCaxFlACFsapQQUMmc+/6ktHq3xTWw3dPV/Vx+OlQzFwpiW+EGBXKJKELkFN2OsZXD89RsiTp6OIGrekSjNy0xILTut1Vm45auxszNrlAeFj7umCqC2evdpBAxU50BN+mWYpPEtXq7GU46D3m6/Vgb7BYMlbeyQl1aS8O9R0srwX0c2Xc2O0nB3vIinyd+HifNlyeGcIdIBGKHihooN41Obgq X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >>> >>> If you prefer vm_normal_page_pud() to be defined and check pud_special() >>> there, I can do that. But again, I don't yet see how that can make a >>> functional difference considering the so far very limited usage of the >>> special bit, and wonder whether we can do that on top when it became >>> necessary (and when we start to have functional requirement of such). >> >> I hope my explanation why pte_special() even exists and how it is used makes >> it clearer. >> >> It's not that much code to handle it like pte_special(), really. I don't >> expect you to teach GUP-slow about vm_normal_page() etc. > > One thing I can do here is I move the pmd_special() check into the existing > vm_normal_page_pmd(), then it'll be a fixup on top of this patch: > > ===8<=== > diff --git a/mm/memory.c b/mm/memory.c > index 288f81a8698e..42674c0748cb 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -672,11 +672,10 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr, > { > unsigned long pfn = pmd_pfn(pmd); > > - /* > - * There is no pmd_special() but there may be special pmds, e.g. > - * in a direct-access (dax) mapping, so let's just replicate the > - * !CONFIG_ARCH_HAS_PTE_SPECIAL case from vm_normal_page() here. > - */ > + /* Currently it's only used for huge pfnmaps */ > + if (unlikely(pmd_special(pmd))) > + return NULL; Better. I'd appreciate a vm_normal_page_pud(), but I guess I have to be the one cleaning up the mess after you. -- Cheers, David / dhildenb