Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752439AbdFSVwR (ORCPT ); Mon, 19 Jun 2017 17:52:17 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:33830 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752087AbdFSVwO (ORCPT ); Mon, 19 Jun 2017 17:52:14 -0400 Date: Tue, 20 Jun 2017 00:52:10 +0300 From: "Kirill A. Shutemov" To: Catalin Marinas Cc: "Kirill A. Shutemov" , Andrew Morton , Vlastimil Babka , Vineet Gupta , Russell King , Will Deacon , Ralf Baechle , "David S. Miller" , "Aneesh Kumar K . V" , Martin Schwidefsky , Heiko Carstens , Andrea Arcangeli , linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ingo Molnar , "H . Peter Anvin" , Thomas Gleixner Subject: Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper Message-ID: <20170619215210.2crwjou3sfdcj73d@node.shutemov.name> References: <20170615145224.66200-1-kirill.shutemov@linux.intel.com> <20170615145224.66200-2-kirill.shutemov@linux.intel.com> <20170619152228.GE3024@e104818-lin.cambridge.arm.com> <20170619160005.wgj4nymtj2nntfll@node.shutemov.name> <20170619170911.GF3024@e104818-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170619170911.GF3024@e104818-lin.cambridge.arm.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2252 Lines: 48 On Mon, Jun 19, 2017 at 06:09:12PM +0100, Catalin Marinas wrote: > On Mon, Jun 19, 2017 at 07:00:05PM +0300, Kirill A. Shutemov wrote: > > On Mon, Jun 19, 2017 at 04:22:29PM +0100, Catalin Marinas wrote: > > > On Thu, Jun 15, 2017 at 05:52:22PM +0300, Kirill A. Shutemov wrote: > > > > We need an atomic way to setup pmd page table entry, avoiding races with > > > > CPU setting dirty/accessed bits. This is required to implement > > > > pmdp_invalidate() that doesn't loose these bits. > > > > > > > > On PAE we have to use cmpxchg8b as we cannot assume what is value of new pmd and > > > > setting it up half-by-half can expose broken corrupted entry to CPU. > > > > > > > > Signed-off-by: Kirill A. Shutemov > > > > Cc: Ingo Molnar > > > > Cc: H. Peter Anvin > > > > Cc: Thomas Gleixner > > > > > > I'll look at this from the arm64 perspective. It would be good if we can > > > have a generic atomic implementation based on cmpxchg64 but I need to > > > look at the details first. > > > > Unfortunately, I'm not sure it's possbile. > > > > The format of a page table is defined per-arch. We cannot assume much about > > it in generic code. > > > > I guess we could make it compile by casting to 'unsigned long', but is it > > useful? > > Every architecture manintainer still has to validate that this assumption > > is valid for the architecture. > > You are right, not much gained in doing this. > > Maybe a stupid question but can we not implement pmdp_invalidate() with > something like pmdp_get_and_clear() (usually reusing the ptep_* > equivalent). Or pmdp_clear_flush() (again, reusing ptep_clear_flush())? > > In my quick grep on pmdp_invalidate, it seems to be followed by > set_pmd_at() or pmd_populate() already and the *pmd value after > mknotpresent isn't any different from 0 to the hardware (at least on > ARM). That's unless Linux expects to see some non-zero value here if > walking the page tables on another CPU. The whole reason to have pmdp_invalidate() in first place is to never make pmd clear in the middle. Otherwise we will get race with MADV_DONTNEED. See ced108037c2a for an example of such race. -- Kirill A. Shutemov