Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932081AbVJYH4E (ORCPT ); Tue, 25 Oct 2005 03:56:04 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932082AbVJYH4E (ORCPT ); Tue, 25 Oct 2005 03:56:04 -0400 Received: from caramon.arm.linux.org.uk ([212.18.232.186]:40722 "EHLO caramon.arm.linux.org.uk") by vger.kernel.org with ESMTP id S932081AbVJYH4D (ORCPT ); Tue, 25 Oct 2005 03:56:03 -0400 Date: Tue, 25 Oct 2005 08:55:56 +0100 From: Russell King To: Nicolas Pitre Cc: Hugh Dickins , Andrew Morton , linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/9] mm: arm ready for split ptlock Message-ID: <20051025075555.GA25020@flint.arm.linux.org.uk> Mail-Followup-To: Nicolas Pitre , Hugh Dickins , Andrew Morton , linux-kernel@vger.kernel.org References: <20051022170240.GA10631@flint.arm.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1662 Lines: 35 On Mon, Oct 24, 2005 at 10:45:04PM -0400, Nicolas Pitre wrote: > On Sat, 22 Oct 2005, Russell King wrote: > > Please contact Nicolas Pitre about that - that was my suggestion, > > but ISTR apparantly the overhead is too high. > > Going through a kernel buffer will simply double the overhead. Let's > suppose it should not be a big enough issue to stop the patch from being > merged though (and it looks cleaner that way). However I'd like for the > WARN_ON((unsigned long)frame & 7) to remain as both the kernel and user > buffers should be 64-bit aligned. The WARN_ON is pointless because we guarantee that the stack is always 64-bit aligned on signal handler setup and return. > I don't see how standard COW could not happen. The only difference with > a true write fault as if we used put_user() is that we bypassed the data > abort vector and the code to get the FAR value. Or am I missing > something? pte_write() just says that the page _may_ be writable. It doesn't say that the MMU is programmed to allow writes. If pte_dirty() doesn't return true, that means that the page is _not_ writable from userspace. If you write to it from kernel mode (without using put_user) you'll bypass the MMU read-only protection and may end up writing to a page owned by two separate processes. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 Serial core - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/