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committed 1418bae1c22x
* transaction committing, but not now as qgroup accounting will be wrong again./* SPDX-License-Identifier: GPL-2.0 *//* * Copyright (C) 2014 Facebook. All rights reserved. *//* * Btrfs qgroup overview * * Btrfs qgroup splits into 3 main part: * 1) Reserve * Reserve metadata/data space for incoming operations * Affect how qgroup limit works * * 2) Trace * Tell btrfs qgroup to trace dirty extents. * * Dirty extents including: * - Newly allocated extents * - Extents going to be deleted (in this trans) * - Extents whose owner is going to be modified * * This is the main part affects whether qgroup numbers will stay * consistent. * Btrfs qgroup can trace clean extents and won't cause any problem, * but it will consume extra CPU time, it should be avoided if possible. * * 3) Account * Btrfs qgroup will updates its numbers, based on dirty extents traced * in previous step. * * Normally at qgroup rescan and transaction commit time. *//* * Special performance optimization for balance. * * For balance, we need to swap subtree of subvolume and reloc trees. * In theory, we need to trace all subtree blocks of both subvolume and reloc * trees, since their owner has changed during such swap. * * However since balance has ensured that both subtrees are containing the * same contents and have the same tree structures, such swap won't cause * qgroup number change. * * But there is a race window between subtree swap and transaction commit, * during that window, if we increase/decrease tree level or merge/split tree * blocks, we still need to trace the original subtrees. * * So for balance, we use a delayed subtree tracing, whose workflow is: * * 1) Record the subtree root block get swapped. * * During subtree swap: * O = Old tree blocks * N = New tree blocks * reloc tree subvolume tree X * Root Root * / \ / \ * NA OB OA OB * / | | \ / | | \ * NC ND OE OF OC OD OE OF * * In this case, NA and OA are going to be swapped, record (NA, OA) into * subvolume tree X. * * 2) After subtree swap. * reloc tree subvolume tree X * Root Root * / \ / \ * OA OB NA OB * / | | \ / | | \ * OC OD OE OF NC ND OE OF * * 3a) COW happens for OB * If we are going to COW tree block OB, we check OB's bytenr against * tree X's swapped_blocks structure. * If it doesn't fit any, nothing will happen. * * 3b) COW happens for NA * Check NA's bytenr against tree X's swapped_blocks, and get a hit. * Then we do subtree scan on both subtrees OA and NA. * Resulting 6 tree blocks to be scanned (OA, OC, OD, NA, NC, ND). * * Then no matter what we do to subvolume tree X, qgroup numbers will