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We currently have three interfaces for attaching existing inodes to normal filesystems(*). - d_add() requires an unhashed or in-lookup dentry and doesn't handle splicing in case a directory already has dentry - d_instantiate() requires a hashed dentry, and also doesn't handle splicing. - d_splice_alias() requires unhashed or in-lookup and does handle splicing, and can return an alternate dentry. So there is no interface that supports both hashed and in-lookup, which is what ->atomic_open needs to deal with. Some filesystems check for in-lookup in their atomic_open and if found, perform a ->lookup and can subsequently use d_instantiate() if the dentry is still negative. Others d_drop() the dentry so they can use d_splice_alias(). This last will cause a problem for proposed changes to locking which require the dentry to remain hashed while an operation proceeds on it. There is also no interface which splices a directory (which might already have a dentry) to a hashed dentry. Filesystems which need to do this d_drop() first. Some filesystems (NFS) skip ->lookup processing for LOOKUP_CREATE|LOOKUP_EXCL which includes mknod, link, symlink etc. So these inode operations might get an unhashed or a hashed-negative dentry. There is no interface for instantiating these so again they need to unhash first (nfs_link) So with this patch d_splice_alias() can handle hashed, unhashed, or in-lookup dentries. This makes it suitable for ->lookup, ->atomic_open, and ->mkdir as well as others. As a side effect d_add() will also now handle hashed dentries, but I have plans to remove d_add() as there is no benefit having it as well as the others. Once updated to handle nr_dentry_negative as is required for hashed dentrties, __d_add() contains code that is identical to __d_instantiate(), so the former is changed to call the later so now: - d_add() calls __d_add() which hashes and might call __d_instantiate. - d_instantiate() calls __d_instantiate() with appropriate locks. It was suggested by Al Viro https://lore.kernel.org/all/20250813050717.GD222315@ZenIV/ that rather than allow d_splice_alias() to handle both hashed and unhashed, we should have a new d_splice_alias_hashed(). I chose not to follow this path because, as noted above, there are several cases where the filesystem has no a priori knowledge of the state of the dentry, and so would need if (d_unhashed(dentry)) alias = d_splice_alias(inode, dentry); else alias = d_splice_alias_hashed(inode, dentry); which is clumsy. Also I hope to minimise the distinction between hashed and in-lookup (they will both be hashed, just with different DCACHE_ENTRY_TYPE) and reduce the use of unhashed dentries. Unhashed dentries would only be created by d_alloc_name() and all of those are passed to d_make_persistent() (though configfs passes some to d_add(dentry, NULL) first!). So the use-case for d_splice_alias() on unhashed dentries would disappear. Note that d_make_persistent() already handles both hashed and unhashed dentries - just not in-lookup. * There is also d_make_persistent() for filesystems which are dcache-based and don't support mkdir, create etc, and d_instantiate_new() for newly created inodes that are still locked. Signed-off-by: NeilBrown <neil@brown.name> Link: https://patch.msgid.link/20260904215142.1060510-3-neilb@ownmail.net Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.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 * 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