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Date: Thu, 10 Dec 2020 10:10:58 -0800 From: "Yu, Yu-cheng" <yu-cheng.yu@...el.com> To: Borislav Petkov <bp@...en8.de> Cc: x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, linux-mm@...ck.org, linux-arch@...r.kernel.org, linux-api@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, Andy Lutomirski <luto@...nel.org>, Balbir Singh <bsingharora@...il.com>, Cyrill Gorcunov <gorcunov@...il.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Eugene Syromiatnikov <esyr@...hat.com>, Florian Weimer <fweimer@...hat.com>, "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, Mike Kravetz <mike.kravetz@...cle.com>, Nadav Amit <nadav.amit@...il.com>, Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>, Peter Zijlstra <peterz@...radead.org>, Randy Dunlap <rdunlap@...radead.org>, "Ravi V. Shankar" <ravi.v.shankar@...el.com>, Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>, Dave Martin <Dave.Martin@....com>, Weijiang Yang <weijiang.yang@...el.com>, Pengfei Xu <pengfei.xu@...el.com> Subject: Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW On 12/10/2020 9:41 AM, Borislav Petkov wrote: > On Tue, Dec 08, 2020 at 11:24:16AM -0800, Yu, Yu-cheng wrote: >> Case (a) is a normal writable data page that has gone through fork(). So it > > Writable > >> has W=0, D=1. But here, the software chooses not to use the D bit, and > > But it has W=0. So not writable? Maybe I will change to: A page in a writable vma, has been modified and gone through fork(). >> instead, W=0, COW=1. > > So the "new" way of denoting that the page is modified is COW=1 > *when* on CET hw. The D=1 bit is still used on the rest thus the two > _PAGE_DIRTY_BITS. > > Am I close? COW=1 is only used in copy-on-write situation (when CET is enabled). If W=1, D bit is used. >> Case (b) is a normal read-only data page. Since it is read-only, fork() >> won't affect it. In __get_user_pages(), a copy of the read-only page is >> needed, and the page is duplicated. The software sets COW=1 for the new >> copy. > > That makes more sense. > >> Thread-A is writing to a writable page, and the page's PTE is becoming W=1, >> D=1. In the middle of it, Thread-B is changing the PTE to W=0. > > Yah, add that to the explanation pls. > Sure.
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