From a68dabd45f3591456ecf7e35f6a6077db79f6bc6 Mon Sep 17 00:00:00 2001 From: "Darrick J. Wong" Date: Wed, 15 Mar 2023 15:59:35 +0100 Subject: [PATCH] xfs: fix off-by-one error in xfs_btree_space_to_height Source kernel commit: c0f399ff51495ac8d30367418f4f6292ecd61fbe 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 incorrect because the first time through the loop it should be using a fanout factor of 2. With this corrected, the above becomes: ht 1 node_blocks 2 blockleft 67108863 ht 2 node_blocks 90 blockleft 67108861 ht 3 node_blocks 4050 blockleft 67108771 ht 4 node_blocks 182250 blockleft 67104721 ht 5 node_blocks 8201250 blockleft 66922471 final height: 6 Fixes: 9ec691205e7d ("xfs: compute the maximum height of the rmap btree when reflink enabled") Signed-off-by: Darrick J. Wong Reviewed-by: Dave Chinner Signed-off-by: Carlos Maiolino Signed-off-by: Pavel Reichl --- libxfs/xfs_btree.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/libxfs/xfs_btree.c b/libxfs/xfs_btree.c index 65d38637..38a3092d 100644 --- a/libxfs/xfs_btree.c +++ b/libxfs/xfs_btree.c @@ -4663,7 +4663,12 @@ 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 fewer 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]. + */ + unsigned long long node_blocks = 2; unsigned long long blocks_left = leaf_blocks - 1; unsigned int height = 1; -- 2.40.0