From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 33DDFC4332F for ; Wed, 28 Dec 2022 20:50:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231472AbiL1Uux (ORCPT ); Wed, 28 Dec 2022 15:50:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34594 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229668AbiL1Uuu (ORCPT ); Wed, 28 Dec 2022 15:50:50 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C746212A8B for ; Wed, 28 Dec 2022 12:50:41 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=f5MIKcljMcE7MvfbzWkOdHywhkXeIvtEXmo6zdl9LQ8=; b=WigkmznklbBaMTp0pO+NktJkdC C0sozicZLpHWh40cEB8nAniR+BhkVcdcHqmITf3ej6X3mp2apCnPArdPZSuWqvV8XvcLX2BUUFgK5 cIZkTMjuE9ZpSIFc7rNTPif1WxDcd8+rxJJDncIeicUjOehoZ0PEHYwxKg0VeZVGr6G2WmsF4j0LF jRUk1EIBgVatyhqKiXQNqmIxrI7AB3nM3tCzMOSIrppuf/rtIxY6cA9PRAR7C4VujPRpsHNWC7Mkv MN8acv8vbQgfbk3ktn8HVX4yZ0XzAhXCME0DPSpicmf43xvVOLRiN2aCx51809WjFOve3PMekmMcB aXO0/VVg==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pAdNk-009Nk8-Gs; Wed, 28 Dec 2022 20:50:36 +0000 Date: Wed, 28 Dec 2022 20:50:36 +0000 From: Matthew Wilcox To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: linux-mm@kvack.org, liam.howlett@oracle.com, surenb@google.com, ldufour@linux.ibm.com, michel@lespinasse.org, vbabka@suse.cz, linux-kernel@vger.kernel.org Subject: Re: [QUESTION] about the maple tree and current status of mmap_lock scalability Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 28, 2022 at 09:48:51PM +0900, Hyeonggon Yoo wrote: > Hello mm folks, > > I have a few questions about the current status of mmap_lock scalability. > > ============================================================= > What is currently causing the kernel to use mmap_lock to protect the maple tree? > ============================================================= > > I understand that the long-term goal is to remove the need for mmap_lock in readers > while traversing the maple tree, using techniques such as RCU or SPF. > What is the biggest obstacle preventing this from being achieved at this time? The long term goal is even larger than this. Ideally, the VMA tree would be protected by a spinlock rather than a mutex. That turned out to be too large a change for the moment (and isn't all that important compared to enabling RCU readers) > ================================================== > How does the maple tree provide RCU-safe manipulation of VMAs? > ================================================== > > Is it similar to the approach suggested in the RCUVM paper (replacing the original > root node with a new root node that shares most of its nodes and deferring > the freeing of stale nodes using RCU)? > > I'm having difficulty understanding the design of the maple tree in this regard. > > [RCUVM paper] https://pdos.csail.mit.edu/papers/rcuvm:asplos12.pdf While I've read the RCUVM paper, I wouldn't say it was particularly an inspiration. The Maple Tree is independent of the VM; it's a general purpose B-tree. As with any B-tree, when modifying a node, we don't touch nodes that we don't need to touch. As with any RCU data structure, we defer freeing things while RCU readers might still have a reference to them. We don't necessarily go all the way to the root node when modifying a leaf node. For example, if we have this structure: Root: Node A, 4000, Node B Node A: p1, 50, p2, 100, p3, 150, p4, 200, NULL, 250, p6, 1000, p7 Node B: p8, 4050, p9, 4100, p10, 4150, p11, 4200, NULL, 4250, p13 and we replace p4 with a NULL over the whole range from 150-199, we construct a new Node A2 that contains: Node A2: p1, 50, p2, 100, p3, 150, NULL, 250, p6, 1000, p7 and we simply write A2 over the entry in Root. Then we mark Node A as dead and RCU-free Node A. There's no need to replace Root as stores to a pointer are atomic. If we need to rebalance between Node A and Node B, we will need to create a new Root (as well as both A and B), mark all of them as dead and RCU-free them.