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The current swap entry allocation/freeing workflow has never had a clear definition. This makes it hard to debug or add new optimizations. This commit introduces a proper definition of how swap entries would be allocated and freed. Now, most operations are folio based, so they will never exceed one swap cluster, and we now have a cleaner border between swap and the rest of mm, making it much easier to follow and debug, especially with new added sanity checks. Also making more optimization possible. Swap entry will be mostly freed and free with a folio bound. The folio lock will be useful for resolving many swap related races. Now swap allocation (except hibernation) always starts with a folio in the swap cache, and gets duped/freed protected by the folio lock: - folio_alloc_swap() - The only allocation entry point now. Context: The folio must be locked. This allocates one or a set of continuous swap slots for a folio and binds them to the folio by adding the folio to the swap cache. The swap slots' swap count start with zero value. - folio_dup_swap() - Increase the swap count of one or more entries. Context: The folio must be locked and in the swap cache. For now, the caller still has to lock the new swap entry owner (e.g., PTL). This increases the ref count of swap entries allocated to a folio. Newly allocated swap slots' count has to be increased by this helper as the folio got unmapped (and swap entries got installed). - folio_put_swap() - Decrease the swap count of one or more entries. Context: The folio must be locked and in the swap cache. For now, the caller still has to lock the new swap entry owner (e.g., PTL). This decreases the ref count of swap entries allocated to a folio. Typically, swapin will decrease the swap count as the folio got installed back and the swap entry got uninstalled This won't remove the folio from the swap cache and free the slot. Lazy freeing of swap cache is helpful for reducing IO. There is already a folio_free_swap() for immediate cache reclaim. This part could be further optimized later. The above locking constraints could be further relaxed when the swap table is fully implemented. Currently dup still needs the caller to lock the swap entry container (e.g. PTL), or a concurrent zap may underflow the swap count. Some swap users need to interact with swap count without involving folio (e.g. forking/zapping the page table or mapping truncate without swapin). In such cases, the caller has to ensure there is no race condition on whatever owns the swap count and use the below helpers: - swap_put_entries_direct() - Decrease the swap count directly. Context: The caller must lock whatever is referencing the slots to avoid a race. Typically the page table zapping or shmem mapping truncate will need to free swap slots directly. If a slot is cached (has a folio bound), this will also try to release the swap cache. - swap_dup_entry_direct() - Increase the swap count directly. Context: The caller must lock whatever is referencing the entries to avoid race, and the entries must already have a swap count > 1. Typically, forking will need to copy the page table and hence needs to increase the swap count of the entries in the table. The page table is locked while referencing the swap entries, so the entries all have a swap count > 1 and can't be freed. Hibernation subsystem is a bit different, so two special wrappers are here: - swap_alloc_hibernation_slot() - Allocate one entry from one device. - swap_free_hibernation_slot() - Free one entry allocated by the above helper. All hibernation entries are exclusive to the hibernation subsystem and should not interact with ordinary swap routines. By separating the workflows, it will be possible to bind folio more tightly with swap cache and get rid of the SWAP_HAS_CACHE as a temporary pin. This commit should not introduce any behavior change [kasong@tencent.com: fix leak, per Chris Mason. Remove WARN_ON, per Lai Yi] Link: https://lkml.kernel.org/r/CAMgjq7AUz10uETVm8ozDWcB3XohkOqf0i33KGrAquvEVvfp5cg@mail.gmail.com [ryncsn@gmail.com: fix KSM copy pages for swapoff, per Chris] Link: https://lkml.kernel.org/r/aXxkANcET3l2Xu6J@KASONG-MC4 Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-14-8862a265a033@tencent.com Signed-off-by: Kairui Song <kasong@tencent.com> Signed-off-by: Kairui Song <ryncsn@gmail.com> Acked-by: Rafael J. Wysocki (Intel) <rafael@kernel.org> Reviewed-by: Baoquan He <bhe@redhat.com> Cc: Baolin Wang <baolin.wang@linux.alibaba.com> Cc: Barry Song <baohua@kernel.org> Cc: Chris Li <chrisl@kernel.org> Cc: Nhat Pham <nphamcs@gmail.com> Cc: Yosry Ahmed <yosry.ahmed@linux.dev> Cc: Deepanshu Kartikey <kartikey406@gmail.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Kairui Song <ryncsn@gmail.com> Cc: Chris Mason <clm@fb.com> Cc: Chris Mason <clm@meta.com> Cc: Lai Yi <yi1.lai@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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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 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