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[209.85.208.170]) by smtp.gmail.com with ESMTPSA id w204sm242736lff.241.2020.12.09.09.58.29 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 09 Dec 2020 09:58:29 -0800 (PST) Received: by mail-lj1-f170.google.com with SMTP id y16so3473163ljk.1 for ; Wed, 09 Dec 2020 09:58:29 -0800 (PST) X-Received: by 2002:a2e:5750:: with SMTP id r16mr1446782ljd.61.1607536708800; Wed, 09 Dec 2020 09:58:28 -0800 (PST) MIME-Version: 1.0 References: <20201209163950.8494-1-will@kernel.org> <20201209163950.8494-2-will@kernel.org> In-Reply-To: <20201209163950.8494-2-will@kernel.org> From: Linus Torvalds Date: Wed, 9 Dec 2020 09:58:12 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting To: Will Deacon Cc: Linux Kernel Mailing List , Linux-MM , Linux ARM , Catalin Marinas , Jan Kara , Minchan Kim , Andrew Morton , "Kirill A . Shutemov" , Vinayak Menon , Android Kernel Team Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 9, 2020 at 8:40 AM Will Deacon wrote: > > @@ -3978,8 +3994,17 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf) > > /* check if the page fault is solved */ > vmf->pte -= (vmf->address >> PAGE_SHIFT) - (address >> PAGE_SHIFT); > - if (!pte_none(*vmf->pte)) > - ret = VM_FAULT_NOPAGE; > + if (pte_none(*vmf->pte)) > + goto out_unlock; > + > + if (vmf->flags & FAULT_FLAG_PREFAULT_OLD) { > + pte_t pte = pte_mkyoung(*vmf->pte); > + if (ptep_set_access_flags(vmf->vma, address, vmf->pte, pte, 0)) > + update_mmu_cache(vmf->vma, address, vmf->pte); > + } Oh, please dear God no. First you incorrectly set it old, and then you conditionally make it young again and as a result force an atomic rwm update and another TLB flush for no good reason. Just make sure that the FAULT_FLAG_PREFAULT_OLD never sets the *actual* address to old. And yes, that probably means that you need to change "alloc_set_pte()" to actually pass in the real address, and leave "vmf->address" alone - so that it can know which ones are prefaulted and which one is real, but that sounds like a good idea anyway. Then you can just make alloc_set_pte() do the right thing in the first place, instead of doing this nasty "lets do it wrong and fix it up later" horror. Linus