surface-camera-build
e968b3964e
A put cost 60-131 ms on the box while a read cost 1-2 ms, and the reason was one call: every write path reached `append` through `device_and_layout()` → `read_image()`, which reads the WHOLE DEVICE — 100,663,296 bytes here — into a KVVec, to append ~70 bytes. `append` itself never looks at the image: it reads the log region and the control block. The read paths had been taught to read only where the image lies; the write paths never were. `AppendSource` now names what an append reads: `whole_image` (kept for the two callers that genuinely need it — a bare store, whose log runs to the end of the file, and the v1→v4 migration, which folds) or `log_head` (a device's control block plus the log's first `WAL_HEADER_LEN` bytes). `write_view()` reads exactly that, `append_mutation()` is the single entry point all four writers use — put, del, the boot record, and the misc device's write_iter — and the bare-image fallback is decided in one place instead of four. The fold still reads the whole store, because a fold rewrites it; that is the honest tail, and it is why a fold belongs on a timer rather than on the path a caller waits behind. The log is bounded by `LOG_FOLD_BYTES`, because it is on the READ path: `Addressed::open` reads `WAL_HEADER_LEN + log_used` bytes on every call, so an unfolded log is a tax on every read, not a bill paid once at the fold. The live store's control block reports log capacity 50,335,744 bytes — if it ever filled, every coordinate read would read ~48 MB before answering anything. A write that finds the log over the line folds first, through `fold_for_headroom`, and then appends to a fresh log; guarded so one append folds at most once, because a fold that did not shrink the log must not loop. The trade is named in the code: the write that crosses the line pays a bounded, rare fold instead of every reader paying an ever-larger log. The timer's comment — "the log grows at roughly a megabyte a day against a 50 MB region, and a fold rewrites the whole image, so folding more often would buy nothing and cost I/O" — is a data workload's arithmetic, where the log is a recovery artefact and nobody reads it. `space_entry`, `find` and `lower_bound` each allocated a fresh KVVec inside their binary-search loop; one buffer per search now. The boot record's one attempt becomes a bounded retry: `ensure_boot_record` claimed the boot with a swap BEFORE it tried, so a single transient failure cost the boot its record silently — which is exactly what happened on the box, where the first client of the boot could not open the store for writing. It now claims one of `BOOT_RECORD_MAX_ATTEMPTS` with a compare-exchange, and parks the count at the ceiling on success. Release moves to 6.19.3-cubelinux0.7: verify-box-preflight.sh now refuses a same-release reinstall, because the default entry boots the release being replaced. Gates, on #81: verify-enum-cost PASS a write is 1.45 ms mean / 7.46 ms worst (new ceilings: 50 ms write, 5 ms read), and a write does not grow with the store verify-file-store PASS 12 writes to a store that is a FILE, all accounted for verify-syscall PASS the store the kernel writes is byte-identical to userspace's
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 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 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