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Inhibit writeback on COW'd extent buffers for the lifetime of the transaction handle, preventing background writeback from setting BTRFS_HEADER_FLAG_WRITTEN and causing unnecessary re-COW. COW amplification occurs when background writeback flushes an extent buffer that a transaction handle is still actively modifying. When lock_extent_buffer_for_io() transitions a buffer from dirty to writeback, it sets BTRFS_HEADER_FLAG_WRITTEN, marking the block as having been persisted to disk at its current bytenr. Once WRITTEN is set, should_cow_block() must either COW the block again or overwrite it in place, both of which are unnecessary overhead when the buffer is still being modified by the same handle that allocated it. By inhibiting background writeback on actively-used buffers, WRITTEN is never set while a transaction handle holds a reference to the buffer, avoiding this overhead entirely. Add an atomic_t writeback_inhibitors counter to struct extent_buffer, which fits in an existing 6-byte hole without increasing struct size. When a buffer is COW'd in btrfs_force_cow_block(), call btrfs_inhibit_eb_writeback() to store the eb in the transaction handle's writeback_inhibited_ebs xarray (keyed by eb->start), take a reference, and increment writeback_inhibitors. The function handles dedup (same eb inhibited twice by the same handle) and replacement (different eb at the same logical address). Allocation failure is graceful: the buffer simply falls back to the pre-existing behavior where it may be written back and re-COW'd. Also inhibit writeback in should_cow_block() when COW is skipped, so that every transaction handle that reuses an already-COW'd buffer also inhibits its writeback. Without this, if handle A COWs a block and inhibits it, and handle B later reuses the same block without inhibiting, handle A's uninhibit on end_transaction leaves the buffer unprotected while handle B is still using it. This ensures all handles that access a COW'd buffer contribute to the inhibitor count, and the buffer remains protected until the last handle releases it. In lock_extent_buffer_for_io(), when writeback_inhibitors is non-zero and the writeback mode is WB_SYNC_NONE, skip the buffer. WB_SYNC_NONE is used by the VM flusher threads for background and periodic writeback, which are the only paths that cause COW amplification by opportunistically writing out dirty extent buffers mid-transaction. Skipping these is safe because the buffers remain dirty in the page cache and will be written out at transaction commit time. WB_SYNC_ALL must always proceed regardless of writeback_inhibitors. This is required for correctness in the fsync path: btrfs_sync_log() writes log tree blocks via filemap_fdatawrite_range() (WB_SYNC_ALL) while the transaction handle that inhibited those same blocks is still active. Without the WB_SYNC_ALL bypass, those inhibited log tree blocks would be silently skipped, resulting in an incomplete log on disk and corruption on replay. btrfs_write_and_wait_transaction() also uses WB_SYNC_ALL via filemap_fdatawrite_range(); for that path, inhibitors are already cleared beforehand, but the bypass ensures correctness regardless. Uninhibit in __btrfs_end_transaction() before atomic_dec(num_writers) to prevent a race where the committer proceeds while buffers are still inhibited. Also uninhibit in btrfs_commit_transaction() before writing and in cleanup_transaction() for the error path. Reviewed-by: Filipe Manana <fdmanana@suse.com> Reviewed-by: Sun YangKai <sunk67188@gmail.com> Reviewed-by: Boris Burkov <boris@bur.io> Signed-off-by: Leo Martins <loemra.dev@gmail.com> Signed-off-by: David Sterba <dsterba@suse.com> |
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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