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Qu Wenruo cacf358322 btrfs: do not force reloc root creation during qgroup_account_snapshot()
[BUG]
When running btrfs/252 with quota enabled through MKFS_OPTIONS="-O quota",
it has a high chance to trigger the following kernel warning and flips
the fs RO:

  BTRFS info (device dm-2): relocating block group 30408704 flags metadata|dup
  ------------[ cut here ]------------
  WARNING: fs/btrfs/extent-tree.c:879 at lookup_inline_extent_backref+0x74b/0x960 [btrfs], CPU#4: btrfs/2173
  CPU: 4 UID: 0 PID: 2173 Comm: btrfs Not tainted 7.2.0-rc6-custom+ #457 PREEMPT(full)  3adc6528fb66f7a55fe1095385818e742f200aab
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
  RIP: 0010:lookup_inline_extent_backref+0x74b/0x960 [btrfs]
  Call Trace:
   <TASK>
   insert_inline_extent_backref+0x7c/0x160 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   __btrfs_inc_extent_ref+0xa9/0x270 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   __btrfs_run_delayed_refs+0x4af/0x11c0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_run_delayed_refs+0x9d/0xf0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   create_pending_snapshot+0x39d/0xf00 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   create_pending_snapshots+0x9b/0xc0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_commit_transaction+0x280/0xeb0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   prepare_to_relocate+0x147/0x200 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   relocate_block_group+0x6b/0x5e0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_relocate_block_group+0x92c/0x2380 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_relocate_chunk+0x3f/0x1a0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_balance+0xa2c/0x19c0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_ioctl+0x2839/0x2d30 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   __x64_sys_ioctl+0x416/0x9a0
   do_syscall_64+0xe1/0x790
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
   </TASK>
  ---[ end trace 0000000000000000 ]---
  BTRFS info (device dm-2): leaf 4593991680 gen 233 total ptrs 175 free space 5953 owner 2
  BTRFS info (device dm-2): refs 3 lock_owner 2173 current 2173
  	item 0 key (166772736 METADATA_ITEM 1) itemoff 16250 itemsize 33
  		extent refs 1 gen 222 flags 2
  		ref#0: tree block backref root 266
         [ Skip the tree dump ]
  	item 174 key (263225344 METADATA_ITEM 0) itemoff 10328 itemsize 33
  		extent refs 1 gen 162 flags 258
  		ref#0: tree block backref root 267
  BTRFS error (device dm-2): extent item not found for insert, bytenr 179847168 num_bytes 16384 parent 4594335744 root_objectid 273 owner 0 offset 0
  BTRFS error (device dm-2): failed to run delayed ref for logical 179847168 num_bytes 16384 type 182 action 1 ref_mod 1: -117

[CAUSE]
The above error is showing that there is a tree reference to a metadata
extent that is no longer there.

With "ref_verify" mount option (requires CONFIG_BTRFS_DEBUG), there is
some extra debug output:

  BTRFS error (device dm-2): dumping block entry [180961280 16384], num_refs 0, metadata 1, from disk 0
  BTRFS error (device dm-2):   root entry 256, num_refs 18446744073709551615
  BTRFS error (device dm-2):   root entry 273, num_refs 18446744073709551615
  BTRFS error (device dm-2):   Ref action 3, root 273, ref_root 273, parent 0, owner 0, offset 0, num_refs 1
     btrfs_force_cow_block+0x129/0x7d0 [btrfs]
     btrfs_cow_block+0x10a/0x250 [btrfs]
     btrfs_search_slot+0x5eb/0xf40 [btrfs]
     btrfs_insert_empty_items+0x3a/0x70 [btrfs]
     insert_with_overflow+0x53/0x130 [btrfs]
     btrfs_insert_dir_item+0x125/0x290 [btrfs]
     btrfs_add_link+0xaa/0x410 [btrfs]
     btrfs_rename+0x5ea/0xcd0 [btrfs]
     btrfs_rename2+0x28/0x60 [btrfs]
     vfs_rename+0x5b2/0xe10
     filename_renameat2+0x244/0x430
     __x64_sys_rename+0x48/0x70
     do_syscall_64+0xe1/0x790
     entry_SYSCALL_64_after_hwframe+0x4b/0x53
  BTRFS error (device dm-2):   Ref action 2, root 273, ref_root 273, parent 0, owner 0, offset 0, num_refs 18446744073709551615
     btrfs_force_cow_block+0x327/0x7d0 [btrfs]
     btrfs_cow_block+0x10a/0x250 [btrfs]
     btrfs_search_slot+0x5eb/0xf40 [btrfs]
     btrfs_lookup_file_extent+0x4d/0x70 [btrfs]
     btrfs_drop_extents+0x151/0xf00 [btrfs]
     insert_reserved_file_extent+0xfe/0x3e0 [btrfs]
     btrfs_finish_one_ordered+0x549/0xc40 [btrfs]
     btrfs_work_helper+0xde/0x350 [btrfs]
     process_one_work+0x198/0x380
     worker_thread+0x1c8/0x330
     kthread+0xee/0x120
     ret_from_fork+0x28f/0x310
     ret_from_fork_asm+0x11/0x20
  BTRFS error (device dm-2):   Ref action 1, root 273, ref_root 0, parent 4594335744, owner 0, offset 0, num_refs 1
     __btrfs_mod_ref+0x1c5/0x2d0 [btrfs]
     btrfs_copy_root+0x262/0x390 [btrfs]
     create_reloc_root+0xb9/0x370 [btrfs]
     btrfs_init_reloc_root+0xb0/0x1b0 [btrfs]
     record_root_in_trans+0xa6/0xd0 [btrfs]
     create_pending_snapshot+0x383/0xf00 [btrfs]
     create_pending_snapshots+0x9b/0xc0 [btrfs]
     btrfs_commit_transaction+0x280/0xeb0 [btrfs]
     prepare_to_relocate+0x147/0x200 [btrfs]
     relocate_block_group+0x6b/0x5e0 [btrfs]
     btrfs_relocate_block_group+0x92c/0x2380 [btrfs]
     btrfs_relocate_chunk+0x3f/0x1a0 [btrfs]
     btrfs_balance+0xa2c/0x19c0 [btrfs]
     btrfs_ioctl+0x2839/0x2d30 [btrfs]
     __x64_sys_ioctl+0x416/0x9a0
     do_syscall_64+0xe1/0x790

The above shows the direct cause, Ref action 3 is the oldest operation,
which shows the tree block is created by COW.  Then ref action 2 shows
it's COWed away, by a metadata update, meaning the tree block is already
released, should not be referred any more.

Then the final one, is trying to create a reloc tree for subvolume 273,
and that reloc root creation is referring to the already dropped tree
block.

The root cause is that, during qgroup_account_snapshot(), we are calling
record_root_in_trans() with "force = true".
So if the root has no reloc root, we will create one, but at that
timing it's already too late.

Normally reloc root should be created before the commit and current
roots diverge, to avoid the same problem we are hitting.

But during relocation initialization, we are committing the current
running transaction, with a new reloc_control attached halfway.

And if qgroup is enabled, the record_root_in_trans() with "force = true"
calls will force reloc root creation even if we do not and should not
create reloc root at that timing.

[FIX]
Do not force reloc root creation during record_root_in_trans() with
"force = true" cases, which is only called by qgroup_account_snapshot().

If we're really under relocation, the reloc root should be created way
early, before the commit and current root diverge.  If the root has no
reloc tree yet, it means we're still initializing the reloc, and do not
need a reloc root.

So skipping the reloc tree creation in qgroup_account_snapshot() should
be safe.

Link: https://bugzilla.suse.com/show_bug.cgi?id=1275740
Fixes: 4d31778aa2 ("btrfs: qgroup: Fix root item corruption when multiple same source snapshots are created with quota enabled")
Assisted-by: LLM (initial analysis, but incorrect conclusion with too many burnt tokens)
Tested-by: Disha Goel <disgoel@linux.ibm.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
2026-09-02 22:19:30 +02:00
arch s390: 2026-08-07 07:41:40 -07:00
block block: stop the timeout timer when releasing a never added disk 2026-07-29 05:23:20 -06:00
certs treewide: fix indentation and whitespace in Kconfig files 2026-05-28 21:24:42 -07:00
crypto crypto: aes - Fix conditions for selecting MAC dependencies 2026-07-09 11:06:22 -04:00
Documentation Including fixes from netfilter. 2026-08-06 11:39:20 -07:00
drivers s390: 2026-08-07 07:41:40 -07:00
fs btrfs: do not force reloc root creation during qgroup_account_snapshot() 2026-09-02 22:19:30 +02:00
include btrfs: use simple booleans for log_commit field in struct btrfs_root 2026-08-07 19:17:18 +02:00
init rust: allow suspicious_runtime_symbol_definitions lint for Rust >= 1.98 2026-06-29 13:02:15 +02:00
io_uring io_uring: preserve task restrictions across exec 2026-07-30 14:40:22 -06:00
ipc kernel-7.2-rc1.misc 2026-06-15 03:05:50 +05:30
kernel BPF fixes: 2026-08-07 08:08:57 -07:00
lib hardening fixes for v7.2-rc6 2026-07-28 13:36:01 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm 17 hotfixes. 15 are cc:stable. 16 are for MM. 2026-08-06 20:29:38 -07:00
net BPF fixes: 2026-08-07 08:08:57 -07:00
rust rust: time: fix as_micros_ceil() to round correctly for negative Delta 2026-07-18 19:07:35 +02:00
samples samples/damon/mtier: error out for zero quota goal target values 2026-08-04 20:02:02 -07:00
scripts kbuild: rpm-pkg: Preserve BTF sections in kernel modules during debuginfo stripping 2026-07-29 16:21:42 -07:00
security selinux/stable-7.2 PR 20260805 2026-08-05 13:40:11 -07:00
sound ALSA: usb-audio: Fix sticky mixer regressions on M-Audio Fast Track Ultra 2026-08-07 10:35:31 +02:00
tools BPF fixes: 2026-08-07 08:08:57 -07:00
usr kbuild: uapi: also test UAPI headers against C++ compilers 2026-03-25 13:24:42 +01:00
virt KVM/arm64 fixes for 7.2, take #2 2026-07-15 12:13:30 +02:00
.clang-format clang-format: fix formatting of guard() and scoped_guard() statements 2026-05-28 21:24:41 -07:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add myself 2026-04-02 16:48:25 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore SPDX patches for 7.2-rc1 2026-06-22 12:06:22 -07:00
.mailmap 17 hotfixes. 15 are cc:stable. 16 are for MM. 2026-08-06 20:29:38 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS CREDITS: Add Wolfram Sang 2026-07-14 16:34:04 +02:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS sound fixes for 7.2-rc7 2026-08-07 06:36:11 -07:00
Makefile Linux 7.2-rc6 2026-08-02 16:24:24 -07:00
README docs: add AI Coding Assistants documentation 2026-01-06 14:55:06 -07:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
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* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

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* Building requirements: Documentation/process/changes.rst
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* License: See COPYING

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https://www.kernel.org/doc/html/latest/


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============

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* New Kernel Developer - Getting started with kernel development
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For Specific Users
==================

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--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
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* Coding Style: Documentation/process/coding-style.rst
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* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
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---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
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-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

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--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
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----------

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* Maintainer Handbook: Documentation/maintainer/index.rst
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---------------

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* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

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-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

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-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


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=========================

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