[PATCH v2 0/4] minix: convert to iomap and add direct I/O

[Jeremy Bingham]

This is v2 of the minix iomap conversion series. The original v1
submission (3 patches) was tested by syzbot, which found four issues.
Three were straightforward; the fourth, a null pointer dereference in
page_symlink when creating symlinks, required more substantial changes.

The original description follows:

This series converts the minix filesystem from the buffer_head-based
I/O path to the iomap API, and adds direct I/O support in the process.

The conversion is straightforward: minix's indirect block tree mapping
logic (from itree_common.c) is reworked into an iomap_begin/iomap_end
implementation. The iomap_end callback is a no-op since minix has no
extents or transactions to finalize.

Patch 1 adds the iomap infrastructure: the new iomap.c file, wrapper
functions and iomap_ops structs in itree_v1.c and itree_v2.c, and the
relevant declarations in minix.h.

The iomap.c file is #include'd into itree_v1.c and itree_v2.c rather
than compiled as a standalone translation unit. This is because the
minix filesystem versions (V1 vs V2/V3) have different block_t sizes
(16-bit vs 32-bit) and different indirect tree depths. This follows
the existing pattern in minix where itree_common.c is included into
both itree_v1.c and itree_v2.c. Each version provides a thin wrapper
and a corresponding iomap_ops struct.

Patch 2 converts the regular file address space operations to iomap:
read_folio, readahead, writepages (with a writeback callback), bmap,
and folio lifecycle helpers. Directory inodes continue to use
buffer_head-based operations via a new minix_dir_aops, since directory
handling still relies on buffer head chunks for prepare/write_begin.

Patch 3 converts the file_operations: replacing the generic read/write
iterators with iomap-aware versions, adding direct I/O read/write paths
using iomap_dio_rw, and setting FMODE_CAN_ODIRECT in the open handler.

The minix iomap implementation was adapted from the out-of-tree xiafs
iomap conversion. The xiafs module itself borrowed heavily from the
modernized minix kernel module. The exfat iomap changes were an
additional reference for both conversions.

Changes since v1:

  * Added a fourth patch to fix the symlink and truncate issues:
    - Replaced page_symlink with a custom __page_symlink that writes
      the target directly to a data block via minix_new_block +
      sb_getblk, bypassing the aops write path (which no longer has
      write_begin/write_end). Added a matching custom minix_get_link
      that reads the target from the data block via sb_bread, similar
      to ext4_get_link. No iomap-based filesystem in the kernel uses
      page_symlink; XFS, GFS2, and ext4 all handle symlink storage
      directly. The on-disk format is unchanged.
    - Fixed a buffer_head/iomap type confusion in truncate:
      block_truncate_page attaches buffer_heads to data folios, but
      minix_aops now uses iomap which interprets folio->private as
      struct iomap_folio_state. truncate() now dispatches between
      iomap_truncate_page (for regular files/symlinks) and
      block_truncate_page (for directories) based on the inode's aops.
    - Added .setattr = minix_setattr to minix_symlink_inode_operations
      so symlinks truncate properly through the iomap path.

  * Patch 1 (iomap infrastructure): minix_get_block is now exported
    (non-static) so the directory aops and iomap writeback path can
    use it. Added minix_iomap_ops_ver() inline helper and extern
    declarations for minix_aops and the version-specific iomap_ops.
    Fixed unsigned -> unsigned int in minix_blocks_needed and
    minix_find_first_zero_bit to silence checkpatch warnings.

  * Patch 2 (aops conversion): unchanged in approach; minor cleanup
    of the writeback callback and minix_bmap conversion.

  * Patch 3 (file operations): minix_setattr is now exported for reuse
    by the symlink inode operations in patch 4.

Testing: the full series has been tested with mkfs.minix V1/V2/V3,
exercising file creation, read/write, overwrite, append, binary data,
directories, symlinks (full path, relative, directory symlinks), hard
links, truncation (shrink/grow), large files (1MB, exercising indirect
blocks), deep nesting (20 levels), 100 files in one directory,
deletions, remount persistence, and fsck.minix. All pass cleanly. The
four syzbot-reported issues are resolved.

Jeremy Bingham (4):
  minix: add iomap infrastructure
  minix: convert address space operations to iomap
  minix: convert file operations to iomap and add direct I/O
  minix: fix symlilnk and truncate for iomap compatibility

 fs/minix/file.c         | 157 ++++++++++++++++++++++++++++++++++++++--
 fs/minix/inode.c        |  90 ++++++++++++++++++++---
 fs/minix/iomap.c        | 114 +++++++++++++++++++++++++++++
 fs/minix/itree_common.c |  11 ++-
 fs/minix/itree_v1.c     |  25 ++++++-
 fs/minix/itree_v2.c     |  17 ++++-
 fs/minix/minix.h        |  30 +++++++-
 fs/minix/namei.c        | 137 ++++++++++++++++++++++++++++++++++-
 8 files changed, 558 insertions(+), 23 deletions(-)
 create mode 100644 fs/minix/iomap.c

-- 
2.47.3