[RFC] fsck.erofs: design discussion for multi-threaded extraction

[Utkal Singh <singhutkal015@gmail.com>]

Hi Gao Xiang and EROFS community,

I have been contributing to erofs-utils since early March [1]. I would like to discuss a design for multi-threaded extraction in fsck.erofs and get feedback before writing more code.

Current state:
  - fsck.erofs is strictly single-threaded; erofs_verify_inode_data()
    serializes all decompression inside erofsfsck_check_inode()
  - lib/workqueue.c already provides erofs_alloc_workqueue(),
    erofs_queue_work(), and erofs_destroy_workqueue(), used by mkfs.erofs
    for multi-threaded compression (cfg.c_mt_workers, --workers=#)
  - Fragment cache was introduced in 1.8.5 (lib/fragments.c)

Proposed design:

  Since EROFS pclusters are independent, decompression can be
  parallelized while file creation and metadata application (chown,
  chmod, utimensat, xattrs) stay serialized in the main thread.

  Pipeline sketch:
    Main thread: inode walk -> erofs_queue_work() -> collect result
                 -> write output + apply metadata
    Worker N:    erofs_verify_inode_data() for one file

  I plan to reuse the existing erofs_workqueue infrastructure and
  follow the --workers=# convention already used in mkfs.erofs.

Design questions I would appreciate guidance on:

  Q1. Is the existing erofs_workqueue sufficient for fsck, or should
      max_jobs be bounded more tightly to control memory pressure for
      large images?

  Q2. For fragment-deduplicated files (fragment cache from 1.8.5),
      should workers share a mutex around fragment reads, or should
      fragment reads remain in the main thread?

  Q3. Is per-file the right parallelism granularity, or would
      per-pcluster be better for large single compressed files?

  Q4. Should fsck follow --workers=# (matching mkfs) or use -T#?

[1] https://lore.kernel.org/linux-erofs/CAGSu4WPCYtq-+hVc-tg_A4u3a3zxnizx7ui7QSO0R8V1DirJSg@mail.gmail.com/

Thanks,
Utkal Singh