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Qu Wenruo ec78575dde btrfs: always wait for ordered extents to avoid OE races
[BUG]
Syzbot reported a bug that there can be conflicting OEs for the same
range:

  BTRFS critical (device loop4): panic in insert_ordered_extent:264: overlapping ordered extents, existing oe file_offset 16384 num_bytes 430080 flags 0x1089, new oe file_offset 16384 num_bytes 430080 flags 0x80 (errno=-17 Object alrea[  179.162726][ T6897] BTRFS critical (device loop4): panic in insert_ordered_extent:264: overlapping ordered extents, existing oe file_offset 16384 num_bytes 430080 flags 0x1089, new oe file_offset 16384 num_bytes 430080 flags 0x80 (errno=-17 Object already exists)
  ------------[ cut here ]------------
  kernel BUG at fs/btrfs/ordered-data.c:264!
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
  RIP: 0010:btrfs_alloc_ordered_extent+0x943/0xad0
  Call Trace:
   <TASK>
   cow_file_range+0x744/0x12a0
   fallback_to_cow+0x5ea/0xa00
   run_delalloc_nocow+0x110c/0x17a0
   btrfs_run_delalloc_range+0xbe4/0x1c20
   writepage_delalloc+0x104d/0x1ba0
   btrfs_writepages+0x1667/0x28b0
   do_writepages+0x338/0x560
   filemap_fdatawrite_range+0x1f2/0x300
   btrfs_fdatawrite_range+0x54/0xf0
   btrfs_direct_write+0x6a0/0xc30
   btrfs_do_write_iter+0x329/0x790
   do_iter_readv_writev+0x624/0x8d0
   vfs_writev+0x34c/0x990
   __se_sys_pwritev2+0x17a/0x2a0
   do_syscall_64+0x174/0x580
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  ---[ end trace 0000000000000000 ]---

[CAUSE]
Since commit ff66fe6662 ("btrfs: fix incorrect buffered IO fallback
for append direct writes"), if the direct IO finished short, we will
revert the isize back to the original one, so that append writes can be
respected during the buffered fallback.

Normally we rely on lock_and_cleanup_extent_if_need() function during
buffered writeback to wait for any existing ordered extents.

But that ordered extent waiting only happens if the start_pos is inside
the isize.
Since we have reverted the isize during failed direct IO, we will not
wait for any ordered extents.

This means we can have a race where the direct IO OE is still in the
tree, finished but not yet removed, then we're inserting the OE for the
buffered write, causing the above crash.

[FIX]
Make the OE wait to be unconditional, to handle the reverted isize
situation.

And since lock_and_cleanup_extent_if_need() now either lock the
extents or return -EAGAIN, also remove the branches that handles
no-extent-locked cases, and rename it to remove the "_if_need" suffix.

The following micro benchmark shows the runtime difference for
btrfs_buffered_write(), doing `xfs_io -f -c "pwrite 0 1m"` workload,
all values are the average runtime in nano seconds.

      function runtime              |   before    |     after
 -----------------------------------+-------------+---------------
 lock_and_cleanup_extent_if_need()  |     58.2    |    183.0
 btrfs_buffered_write()             |   2115.6    |   2973.3

The overall runtime of btrfs_buffered_write() is still pretty
tiny (still less than 3 micro seconds), I'd say the extra cost is still
acceptable.

An alternative to fix this problem is to wait ordered extents during
iomap_end() where the isize revert is done.

But that solution will break nowait requirement, as if a nowait direct
IO finished short, we have to wait for the OEs unconditionally or the
next append buffered IO can still hit the same problem.

So here we have to move the wait cost to buffered write, but at least
the code is slightly more streamline.

Reported-by: syzbot+ba2afde329fc27e3f22e@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=ba2afde329fc27e3f22e
Fixes: ff66fe6662 ("btrfs: fix incorrect buffered IO fallback for append direct writes")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
History 2026-08-07 19:17:16 +02:00
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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
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

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

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros
* AI Coding Assistant - LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* 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
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

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

System Administrator
--------------------

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
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

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

AI Coding Assistant
-------------------

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.


Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst