Re: [PATCH v6 00/12] f2fs: introduce metadata cache

[Jaegeuk Kim <jaegeuk@kernel.org>]

Chao, can you take a look at these patches?

https://github.com/Sys-AI-Foundation/f2fs/commit/e22a482931fb57d8e093e6cc5617f1f0a35539f5
https://github.com/Sys-AI-Foundation/f2fs/commit/902febb3650c6b5d63bc2d1f1e4a0b28b0796757
https://github.com/Sys-AI-Foundation/f2fs/commit/4a7dd5420623701c12c814f74a1211a5f86bd89b
https://github.com/Sys-AI-Foundation/f2fs/commit/f609f4b93bd68e3caabdfaf7442bd196bfca4d21

On 09/09, Chao Yu wrote:
> This patchset introduces a self-managed metadata block cache in f2fs,
> decoupling meta blocks, node blocks, and compressed data blocks from
> the Linux VFS page cache and fake internal inodes.
> 
> === 1. Background & Motivation ===
> 
> Currently, F2FS uses fake VFS inodes (meta_inode, node_inode, and
> compress_inode) to manage internal block caching through the VFS page
> cache. Because of this implementation, the f2fs block size was
> historically coupled to the kernel page size.
> 
> We now want to unbind block size from page size to support configurations
> where block size <= PAGE_SIZE (e.g., mounting a 4KB-block F2FS image on a
> 16KB or 64KB page system).
> 
> One possible approach is to continue using the VFS page cache to store
> metadata blocks. However, doing so introduces three major architectural
> issues (illustrated by a 4KB block on a 16KB page system):
> 
> 1. Memory Overhead:
>    Metadata access patterns are typically random and sparse. Caching a
>    single 4KB metadata block inside a page cache folio forces the
>    allocation of an entire 16KB folio, resulting in 4x memory waste.
> 
> 2. Folio and Sub-block Conversion Complexity:
>    Using larger folios requires tracking individual sub-block dirty/uptodate
>    states within each folio and performing index-to-offset conversions across
>    function boundaries. Because core metadata structures (e.g., f2fs_checkpoint,
>    f2fs_sit_block, f2fs_nat_block, f2fs_summary_block, f2fs_node) are accessed
>    extensively throughout the filesystem, this sub-block management and offset
>    calculation complexity would spread across the entire F2FS codebase.
> 
> 3. Lock Contention:
>    Multiple independent node blocks (e.g., dnode blocks belonging to
>    different files) can reside within the same folio. Concurrent fsync()
>    calls on unrelated files would contend on the same folio_lock(),
>    serializing metadata updates and degrading multi-threaded performance.
> 
> Decoupling metadata caching from PAGE_SIZE by allocating exact
> block-sized cache entries is the critical first step toward supporting
> 4KB-block F2FS images on 16KB/64KB page systems.
> 
> === 2. Metadata Cache Architecture & Design ===
> 
> This patchset introduces a dedicated, block-size-aligned caching
> infrastructure with the following key components:
> 
> - Block-Size Aligned Allocation:
>   Allocates memory buffers matching exactly the filesystem block size
>   (4KB or 16KB) via kzalloc(), fully independent of the host
>   architecture's PAGE_SIZE.
> 
> - Radix Tree Indexing with Fast Tag Scanning:
>   Each cache instance (META_CACHE, NODE_CACHE, COMPRESS_CACHE) indexes
>   cached blocks via a radix tree (keyed by Physical Block Address for meta/
>   compress cache, and Node ID for node cache). Radix tree tags
>   (F2FS_CACHE_TAG_DIRTY, F2FS_CACHE_TAG_WRITEBACK) provide O(1) batch gang
>   lookups for flushing and writeback without dual-list shuffling.
> 
> - Lightweight Bit-Locking:
>   Individual entries use atomic bit locks (F2FS_BLOCK_LOCKED via
>   wait_on_bit_lock() / clear_and_wake_up_bit()) rather than heavyweight
>   embedded mutexes/semaphores, minimizing memory footprint per entry.
> 
> - Direct BIO Read/Write & BIO Merging:
>   Decouples metadata/node I/O from VFS address spaces by submitting direct
>   BIOs (f2fs_submit_cache_read / f2fs_submit_cache_write) with chained
>   adjacent vector merging (entry->next_entry) and dedicated completion
>   handlers.
> 
> - Memory Reclamation Shrinker:
>   Integrates with the kernel shrinker subsystem via a 3-phase isolation
>   algorithm (isolate unreferenced clean entries -> truncate from radix tree
>   under lock -> splice un-reclaimed entries back to LRU) to safely reclaim
>   clean cached blocks under system memory pressure.
> 
> - Background Writeback Kthread & Checkpoint Integration:
>   Provides a dedicated background kthread (f2fs_writeback-X:Y) for periodic
>   dirty cache flushing, combined with synchronous flushing during checkpoint
>   commit.
> 
> - Fault Injection, Tracepoints & Debugfs Observability:
>   Integrates FAULT_KALLOC fault injection, tracepoints for cache state
>   transitions and batch writeback, and per-cache memory breakdowns in debugfs.
> 
> === 3. Patchset Organization ===
> 
> - Patch 01: Implement the core metadata cache infrastructure & direct BIO I/O.
> - Patch 02: Initialize and teardown META_CACHE in sb_info.
> - Patch 03: Integrate metadata cache into the memory shrinker subsystem.
> - Patch 04: Introduce the background writeback kernel thread.
> - Patch 05: Migrate metadata block caching (SIT, NAT, SSA, CP, recovery, GC)
>             from meta_inode to META_CACHE.
> - Patch 06: Initialize and teardown NODE_CACHE in sb_info.
> - Patch 07: Migrate node and inode block caching from node_inode to NODE_CACHE.
> - Patch 08: Initialize and teardown COMPRESS_CACHE in sb_info.
> - Patch 09: Migrate compressed cluster caching from compress_inode to COMPRESS_CACHE.
> - Patch 10: Add fault injection support for cache allocation paths.
> - Patch 11: Introduce ftrace tracepoints for cache dirty and writeback events.
> - Patch 12: Expose per-cache memory usage in debugfs.
> 
> Changelog:
> 
> v5->v6:
> - rebase on last dev-test
> - rely on ("f2fs: parameterize node helpers and macros")
> - remove unneeded f2fs_cache_test_dirty() check in f2fs_mark_cache_dirty()
> - drop f2fs_force_clear_cache_dirty()
> - avoid unnecessary type cast before cache_address() in checkpoint.c
> - fix to init cache_wb_interval in prior create f2fs_writeback kthread
> - fix to goto restore_wb label if f2fs_sync_fs() failed in f2fs_remount()
> - fix to avoid race condition when access entry->ino in between
>   f2fs_cache_compressed_page() and f2fs_invalidate_compress_pages()
> - rebase on decoupling patchset from Kelvin
> - fix to access IS_INODE() and ino_of_inode() under cache lock
> 
> Chao Yu (12):
>   f2fs: cache: implement metadata cache
>   f2fs: cache: initialize meta cache
>   f2fs: cache: introduce shrinker
>   f2fs: cache: introduce writeback thread
>   f2fs: cache: use meta cache
>   f2fs: cache: initialize node cache
>   f2fs: cache: use node cache
>   f2fs: cache: initialize compress cache
>   f2fs: cache: use compress cache
>   f2fs: cache: support fault injection
>   f2fs: cache: introduce tracepoints
>   f2fs: cache: show per-cache usage in debugfs
> 
>  Documentation/ABI/testing/sysfs-fs-f2fs |    6 +
>  fs/f2fs/Makefile                        |    2 +-
>  fs/f2fs/acl.c                           |   26 +-
>  fs/f2fs/acl.h                           |    8 +-
>  fs/f2fs/cache.c                         |  704 ++++++++++++++++
>  fs/f2fs/cache.h                         |  242 ++++++
>  fs/f2fs/checkpoint.c                    |  397 ++++-----
>  fs/f2fs/compress.c                      |  163 ++--
>  fs/f2fs/data.c                          |  520 ++++++++----
>  fs/f2fs/debug.c                         |   70 +-
>  fs/f2fs/dir.c                           |  170 ++--
>  fs/f2fs/extent_cache.c                  |   14 +-
>  fs/f2fs/f2fs.h                          |  361 ++++----
>  fs/f2fs/file.c                          |   78 +-
>  fs/f2fs/gc.c                            |  185 ++--
>  fs/f2fs/inline.c                        |  284 +++----
>  fs/f2fs/inode.c                         |  205 ++---
>  fs/f2fs/iostat.h                        |   11 +
>  fs/f2fs/namei.c                         |  114 +--
>  fs/f2fs/node.c                          | 1023 +++++++++++------------
>  fs/f2fs/node.h                          |  104 +--
>  fs/f2fs/recovery.c                      |  251 +++---
>  fs/f2fs/segment.c                       |  261 +++---
>  fs/f2fs/segment.h                       |   37 +-
>  fs/f2fs/shrinker.c                      |   14 +
>  fs/f2fs/super.c                         |  145 ++--
>  fs/f2fs/sysfs.c                         |   13 +-
>  fs/f2fs/xattr.c                         |  123 ++-
>  fs/f2fs/xattr.h                         |   12 +-
>  include/linux/f2fs_fs.h                 |    3 -
>  include/trace/events/f2fs.h             |   71 ++
>  31 files changed, 3401 insertions(+), 2216 deletions(-)
>  create mode 100644 fs/f2fs/cache.c
>  create mode 100644 fs/f2fs/cache.h
> 
> -- 
> 2.49.0
>