Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753097Ab1FRVsx (ORCPT ); Sat, 18 Jun 2011 17:48:53 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:43452 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752741Ab1FRVsw (ORCPT ); Sat, 18 Jun 2011 17:48:52 -0400 Date: Sat, 18 Jun 2011 14:48:32 -0700 From: Andrew Morton To: Hugh Dickins Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 1/12] radix_tree: exceptional entries and indices Message-Id: <20110618144832.cfc665b0.akpm@linux-foundation.org> In-Reply-To: References: <20110617163854.49225203.akpm@linux-foundation.org> X-Mailer: Sylpheed 2.7.1 (GTK+ 2.18.9; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1994 Lines: 42 On Fri, 17 Jun 2011 17:13:38 -0700 (PDT) Hugh Dickins wrote: > On Fri, 17 Jun 2011, Andrew Morton wrote: > > On Tue, 14 Jun 2011 03:42:27 -0700 (PDT) > > Hugh Dickins wrote: > > > > > The low bit of a radix_tree entry is already used to denote an indirect > > > pointer, for internal use, and the unlikely radix_tree_deref_retry() case. > > > Define the next bit as denoting an exceptional entry, and supply inline > > > functions radix_tree_exception() to return non-0 in either unlikely case, > > > and radix_tree_exceptional_entry() to return non-0 in the second case. > > > > Yes, the RADIX_TREE_INDIRECT_PTR hack is internal-use-only, and doesn't > > operate on (and hence doesn't corrupt) client-provided items. > > > > This patch uses bit 1 and uses it against client items, so for > > practical purpoese it can only be used when the client is storing > > addresses. And it needs new APIs to access that flag. > > > > All a bit ugly. Why not just add another tag for this? Or reuse an > > existing tag if the current tags aren't all used for these types of > > pages? > > I couldn't see how to use tags without losing the "lockless" lookups: So lockless pagecache broke the radix-tree tag-versus-item coherency as well as the address_space nrpages-vs-radix-tree coherency. Isn't it fun learning these things. > because the tag is a separate bit from the entry itself, unless you're > under tree_lock, there would be races when changing from page pointer > to swap entry or back, when slot was updated but tag not or vice versa. So... take tree_lock? What effect does that have? It'd better be "really bad", because this patchset does nothing at all to improve core MM maintainability :( -- 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/