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 5101DC4332F for ; Tue, 20 Dec 2022 16:10:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229724AbiLTQKv (ORCPT ); Tue, 20 Dec 2022 11:10:51 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54906 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233989AbiLTQK1 (ORCPT ); Tue, 20 Dec 2022 11:10:27 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BCC0F17AB2 for ; Tue, 20 Dec 2022 08:10:11 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 62F50B81193 for ; Tue, 20 Dec 2022 16:10:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 18ED4C433EF; Tue, 20 Dec 2022 16:10:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1671552609; bh=YfBzsKkXVIBO2a7ym95eqIQa10CA2D+kGGaVCTFk2X4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=B5nvd5IxslElz/Id3L7sax+RiCIEaMyojW039DjFqBijoFp394vtZ83ObUMqaZGoo yz2MQz4S9XSzAMiFVmaB6RDo3evJMJ7X4EATJR7bP0oDcM/7CXQo9ySeAhDPvv0anV MkuT2AQcn66SgCqbmvHqYjeQFxcYa+ZDBzbIcTg7aPK9efXvYjf5ThfgSqyblExkD4 EZaPsbDaHPs0wrenvcIw8JbX5XCjrhvLvC53uKn9anWClQBjWpHzi6vRgAcej2D9rn YgwajRXCI/FwcE/YwvfjBuahOneAKKCNHR1FyNAtxiRnpzNzkgfcA2lMalsfWJmATs UwsCitivsRmlQ== Date: Tue, 20 Dec 2022 08:10:08 -0800 From: "Darrick J. Wong" To: Dave Chinner Cc: linux-xfs@vger.kernel.org Subject: Re: [PATCH 4/4] xfs: fix off-by-one error in xfs_btree_space_to_height Message-ID: References: <167149469744.336919.13748690081866673267.stgit@magnolia> <167149471987.336919.3277522603824048839.stgit@magnolia> <20221220050001.GK1971568@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221220050001.GK1971568@dread.disaster.area> Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Tue, Dec 20, 2022 at 04:00:01PM +1100, Dave Chinner wrote: > On Mon, Dec 19, 2022 at 04:05:19PM -0800, Darrick J. Wong wrote: > > From: Darrick J. Wong > > > > Lately I've been stress-testing extreme-sized rmap btrees by using the > > (new) xfs_db bmap_inflate command to clone bmbt mappings billions of > > times and then using xfs_repair to build new rmap and refcount btrees. > > This of course is /much/ faster than actually FICLONEing a file billions > > of times. > > > > Unfortunately, xfs_repair fails in xfs_btree_bload_compute_geometry with > > EOVERFLOW, which indicates that xfs_mount.m_rmap_maxlevels is not > > sufficiently large for the test scenario. For a 1TB filesystem (~67 > > million AG blocks, 4 AGs) the btheight command reports: > > > > $ xfs_db -c 'btheight -n 4400801200 -w min rmapbt' /dev/sda > > rmapbt: worst case per 4096-byte block: 84 records (leaf) / 45 keyptrs (node) > > level 0: 4400801200 records, 52390491 blocks > > level 1: 52390491 records, 1164234 blocks > > level 2: 1164234 records, 25872 blocks > > level 3: 25872 records, 575 blocks > > level 4: 575 records, 13 blocks > > level 5: 13 records, 1 block > > 6 levels, 53581186 blocks total > > > > The AG is sufficiently large to build this rmap btree. Unfortunately, > > m_rmap_maxlevels is 5. Augmenting the loop in the space->height > > function to report height, node blocks, and blocks remaining produces > > this: > > > > ht 1 node_blocks 45 blockleft 67108863 > > ht 2 node_blocks 2025 blockleft 67108818 > > ht 3 node_blocks 91125 blockleft 67106793 > > ht 4 node_blocks 4100625 blockleft 67015668 > > final height: 5 > > > > The goal of this function is to compute the maximum height btree that > > can be stored in the given number of ondisk fsblocks. Starting with the > > top level of the tree, each iteration through the loop adds the fanout > > factor of the next level down until we run out of blocks. IOWs, maximum > > height is achieved by using the smallest fanout factor that can apply > > to that level. > > > > However, the loop setup is not correct. Top level btree blocks are > > allowed to contain fewer than minrecs items, so the computation is > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > Ah, that's the critical piece of information I was looking for. I > couldn't work out from the code change below what was wrong with > limits[1]. So.... > > > diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c > > index 4c16c8c31fcb..8d11d3f5e529 100644 > > --- a/fs/xfs/libxfs/xfs_btree.c > > +++ b/fs/xfs/libxfs/xfs_btree.c > > @@ -4666,7 +4666,11 @@ xfs_btree_space_to_height( > > const unsigned int *limits, > > unsigned long long leaf_blocks) > > { > > - unsigned long long node_blocks = limits[1]; > > + /* > > + * The root btree block can have a fanout between 2 and maxrecs because > > + * the tree might not be big enough to fill it. > > + */ > > Can you change this comment to say something like: > > /* > * The root btree block can have less than minrecs pointers > * in it because the tree might not be big enough to require > * that amount of fanout. Hence it has a minimum size of > * 2 pointers, not limits[1]. > */ Done. Thanks for the reviews! :) --D > > Otherwise it looks good. > > Reviewed-by: Dave Chinner > > > + unsigned long long node_blocks = 2; > > unsigned long long blocks_left = leaf_blocks - 1; > > unsigned int height = 1; > > For future consideration, we don't use maxrecs in this calculation > at all - should we just pass minrecs into the function rather than > an array of limits? > > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com