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Thu, 17 Feb 2022 17:58:54 -0800 (PST) X-Google-Smtp-Source: ABdhPJxeJOFDT/yHds7fZ6sSawpHZg5BF2GsuU/gtrCjzQTB2OjN6ABNZJVU5nNeaAO4TkJYyBqRNg== X-Received: by 2002:a17:90a:c292:b0:1b8:d40c:9292 with SMTP id f18-20020a17090ac29200b001b8d40c9292mr6000148pjt.139.1645149534070; Thu, 17 Feb 2022 17:58:54 -0800 (PST) Received: from xz-m1.local ([94.177.118.47]) by smtp.gmail.com with ESMTPSA id c68sm9017323pga.1.2022.02.17.17.58.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 17 Feb 2022 17:58:53 -0800 (PST) Date: Fri, 18 Feb 2022 09:58:48 +0800 From: Peter Xu To: Nadav Amit Cc: Andrew Morton , linux-mm@kvack.org, Nadav Amit , Mike Rapoport , Andrea Arcangeli , stable@vger.kernel.org Subject: Re: [PATCH] userfaultfd: mark uffd_wp regardless of VM_WRITE flag Message-ID: References: <20220217211602.2769-1-namit@vmware.com> MIME-Version: 1.0 In-Reply-To: <20220217211602.2769-1-namit@vmware.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 5287B100004 X-Rspam-User: Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=S0OzyKEJ; spf=none (imf14.hostedemail.com: domain of peterx@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Stat-Signature: obhnzmqcq4h7qn9hj4sm1nrgbmdmogye X-HE-Tag: 1645149537-587982 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: Hello, Nadav, On Thu, Feb 17, 2022 at 09:16:02PM +0000, Nadav Amit wrote: > From: Nadav Amit > > When a PTE is set by UFFD operations such as UFFDIO_COPY, the PTE is > currently only marked as write-protected if the VMA has VM_WRITE flag > set. This seems incorrect or at least would be unexpected by the users. > > Consider the following sequence of operations that are being performed > on a certain page: > > mprotect(PROT_READ) > UFFDIO_COPY(UFFDIO_COPY_MODE_WP) > mprotect(PROT_READ|PROT_WRITE) No objection to the patch, however I'm wondering why this is a valid use case because mprotect seems to be conflict with uffd, because AFAICT mprotect(PROT_READ|PROT_WRITE) can already grant write bit. In change_pte_range(): if (dirty_accountable && pte_dirty(ptent) && (pte_soft_dirty(ptent) || !(vma->vm_flags & VM_SOFTDIRTY))) { ptent = pte_mkwrite(ptent); } PS: I always think here the VM_SOFTDIRTY check is wrong, IMHO it should be: if (dirty_accountable && pte_dirty(ptent) && (pte_soft_dirty(ptent) || (vma->vm_flags & VM_SOFTDIRTY))) { ptent = pte_mkwrite(ptent); } Because when VM_SOFTDIRTY is cleared it means soft dirty enabled. I wanted to post a patch but I never yet. Could I ask why you need mprotect() with uffd? Thanks, -- Peter Xu