Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751144AbdFSRJU (ORCPT ); Mon, 19 Jun 2017 13:09:20 -0400 Received: from foss.arm.com ([217.140.101.70]:54796 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750979AbdFSRJS (ORCPT ); Mon, 19 Jun 2017 13:09:18 -0400 Date: Mon, 19 Jun 2017 18:09:12 +0100 From: Catalin Marinas To: "Kirill A. Shutemov" 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: <20170619170911.GF3024@e104818-lin.cambridge.arm.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170619160005.wgj4nymtj2nntfll@node.shutemov.name> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2501 Lines: 66 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. > > > +static inline pmd_t pmdp_establish(pmd_t *pmdp, pmd_t pmd) > > > +{ > > > + pmd_t old; > > > + > > > + /* > > > + * We cannot assume what is value of pmd here, so there's no easy way > > > + * to set if half by half. We have to fall back to cmpxchg64. > > > + */ > > > + { > > > > BTW, you are missing a "do" here (and it probably compiles just fine > > without it, though different behaviour). > > Ouch. Thanks. > > Hm, what is semantics of the construct without a "do"? You can just ignore the brackets: old = *pmdp; while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd) ; -- Catalin