From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas Subject: Re: [RFC PATCH V4 6/7] arm64: mm: Enable HAVE_RCU_TABLE_FREE logic Date: Wed, 30 Apr 2014 16:20:47 +0100 Message-ID: <20140430152047.GF31220@arm.com> References: <1396018892-6773-1-git-send-email-steve.capper@linaro.org> <1396018892-6773-7-git-send-email-steve.capper@linaro.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from fw-tnat.austin.arm.com ([217.140.110.23]:20295 "EHLO collaborate-mta1.arm.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1422649AbaD3PVe (ORCPT ); Wed, 30 Apr 2014 11:21:34 -0400 Content-Disposition: inline In-Reply-To: <1396018892-6773-7-git-send-email-steve.capper@linaro.org> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Steve Capper Cc: "linux-arm-kernel@lists.infradead.org" , "linux@arm.linux.org.uk" , "linux-mm@kvack.org" , "linux-arch@vger.kernel.org" , "peterz@infradead.org" , "gary.robertson@linaro.org" , "anders.roxell@linaro.org" , "akpm@linux-foundation.org" On Fri, Mar 28, 2014 at 03:01:31PM +0000, Steve Capper wrote: > In order to implement fast_get_user_pages we need to ensure that the > page table walker is protected from page table pages being freed from > under it. > > This patch enables HAVE_RCU_TABLE_FREE, any page table pages belonging > to address spaces with multiple users will be call_rcu_sched freed. > Meaning that disabling interrupts will block the free and protect the > fast gup page walker. > > Signed-off-by: Steve Capper While this patch is simple, I'd like to better understand the reason for it. Currently HAVE_RCU_TABLE_FREE is enabled for powerpc and sparc while __get_user_pages_fast() is supported by a few other architectures that don't select HAVE_RCU_TABLE_FREE. So why do we need it for fast gup on arm/arm64 while not all the other archs need it? Thanks. -- Catalin