From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f171.google.com (mail-pd0-f171.google.com [209.85.192.171]) by kanga.kvack.org (Postfix) with ESMTP id 220DA6B0035 for ; Wed, 30 Apr 2014 13:21:57 -0400 (EDT) Received: by mail-pd0-f171.google.com with SMTP id r10so2067636pdi.2 for ; Wed, 30 Apr 2014 10:21:56 -0700 (PDT) Received: from collaborate-mta1.arm.com (fw-tnat.austin.arm.com. [217.140.110.23]) by mx.google.com with ESMTP id ho7si17641930pad.151.2014.04.30.10.21.55 for ; Wed, 30 Apr 2014 10:21:55 -0700 (PDT) Date: Wed, 30 Apr 2014 18:21:14 +0100 From: Catalin Marinas Subject: Re: [RFC PATCH V4 6/7] arm64: mm: Enable HAVE_RCU_TABLE_FREE logic Message-ID: <20140430172114.GI31220@arm.com> References: <1396018892-6773-1-git-send-email-steve.capper@linaro.org> <1396018892-6773-7-git-send-email-steve.capper@linaro.org> <20140430152047.GF31220@arm.com> <20140430153317.GG31220@arm.com> <20140430153824.GA7166@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140430153824.GA7166@linaro.org> Sender: owner-linux-mm@kvack.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 Wed, Apr 30, 2014 at 04:38:25PM +0100, Steve Capper wrote: > On Wed, Apr 30, 2014 at 04:33:17PM +0100, Catalin Marinas wrote: > > On Wed, Apr 30, 2014 at 04:20:47PM +0100, Catalin Marinas wrote: > > > 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? > > > > OK, replying to myself. I assume the other architectures that don't need > > HAVE_RCU_TABLE_FREE use IPI for TLB shootdown, hence they gup_fast > > synchronisation for free. > > Yes that is roughly the case. > Essentially we want to RCU free the page table backing pages at a > later time when we aren't walking on them. > > Other arches use IPI, some others have their own RCU logic. I opted to > activate some existing logic to reduce code duplication. Both powerpc and sparc use tlb_remove_table() via their __pte_free_tlb() etc. which implies an IPI for synchronisation if mm_users > 1. For gup_fast we may not need it since we use the RCU for protection. Am I missing anything? -- Catalin -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org