Merge tag 'f2fs-for-6.13-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs

Pull f2fs updates from Jaegeuk Kim:
 "This series introduces a device aliasing feature where user can carve
  out partitions but reclaim the space back by deleting aliased file in
  root dir.

  In addition to that, there're numerous minor bug fixes in zoned device
  support, checkpoint=disable, extent cache management, fiemap, and
  lazytime mount option. The full list of noticeable changes can be
  found below.

  Enhancements:
   - introduce device aliasing file
   - add stats in debugfs to show multiple devices
   - add a sysfs node to limit max read extent count per-inode
   - modify f2fs_is_checkpoint_ready logic to allow more data to be
     written with the CP disable
   - decrease spare area for pinned files for zoned devices

  Fixes:
   - Revert "f2fs: remove unreachable lazytime mount option parsing"
   - adjust unusable cap before checkpoint=disable mode
   - fix to drop all discards after creating snapshot on lvm device
   - fix to shrink read extent node in batches
   - fix changing cursegs if recovery fails on zoned device
   - fix to adjust appropriate length for fiemap
   - fix fiemap failure issue when page size is 16KB
   - fix to avoid forcing direct write to use buffered IO on inline_data
     inode
   - fix to map blocks correctly for direct write
   - fix to account dirty data in __get_secs_required()
   - fix null-ptr-deref in f2fs_submit_page_bio()
   - fix inconsistent update of i_blocks in release_compress_blocks and
     reserve_compress_blocks"

* tag 'f2fs-for-6.13-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs: (40 commits)
  f2fs: fix to drop all discards after creating snapshot on lvm device
  f2fs: add a sysfs node to limit max read extent count per-inode
  f2fs: fix to shrink read extent node in batches
  f2fs: print message if fscorrupted was found in f2fs_new_node_page()
  f2fs: clear SBI_POR_DOING before initing inmem curseg
  f2fs: fix changing cursegs if recovery fails on zoned device
  f2fs: adjust unusable cap before checkpoint=disable mode
  f2fs: fix to requery extent which cross boundary of inquiry
  f2fs: fix to adjust appropriate length for fiemap
  f2fs: clean up w/ F2FS_{BLK_TO_BYTES,BTYES_TO_BLK}
  f2fs: fix to do cast in F2FS_{BLK_TO_BYTES, BTYES_TO_BLK} to avoid overflow
  f2fs: replace deprecated strcpy with strscpy
  Revert "f2fs: remove unreachable lazytime mount option parsing"
  f2fs: fix to avoid forcing direct write to use buffered IO on inline_data inode
  f2fs: fix to map blocks correctly for direct write
  f2fs: fix race in concurrent f2fs_stop_gc_thread
  f2fs: fix fiemap failure issue when page size is 16KB
  f2fs: remove redundant atomic file check in defragment
  f2fs: fix to convert log type to segment data type correctly
  f2fs: clean up the unused variable additional_reserved_segments
  ...

1 files changed, 44 insertions, 0 deletions

diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
index 68a0885fb5e6..fb7d2ee022bc 100644
--- a/Documentation/filesystems/f2fs.rst
+++ b/Documentation/filesystems/f2fs.rst
@@ -943,3 +943,47 @@ NVMe Zoned Namespace devices
   can start before the zone-capacity and span across zone-capacity boundary.
   Such spanning segments are also considered as usable segments. All blocks
   past the zone-capacity are considered unusable in these segments.
+
+Device aliasing feature
+-----------------------
+
+f2fs can utilize a special file called a "device aliasing file." This file allows
+the entire storage device to be mapped with a single, large extent, not using
+the usual f2fs node structures. This mapped area is pinned and primarily intended
+for holding the space.
+
+Essentially, this mechanism allows a portion of the f2fs area to be temporarily
+reserved and used by another filesystem or for different purposes. Once that
+external usage is complete, the device aliasing file can be deleted, releasing
+the reserved space back to F2FS for its own use.
+
+<use-case>
+
+# ls /dev/vd*
+/dev/vdb (32GB) /dev/vdc (32GB)
+# mkfs.ext4 /dev/vdc
+# mkfs.f2fs -c /dev/vdc@vdc.file /dev/vdb
+# mount /dev/vdb /mnt/f2fs
+# ls -l /mnt/f2fs
+vdc.file
+# df -h
+/dev/vdb                            64G   33G   32G  52% /mnt/f2fs
+
+# mount -o loop /dev/vdc /mnt/ext4
+# df -h
+/dev/vdb                            64G   33G   32G  52% /mnt/f2fs
+/dev/loop7                          32G   24K   30G   1% /mnt/ext4
+# umount /mnt/ext4
+
+# f2fs_io getflags /mnt/f2fs/vdc.file
+get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable
+# f2fs_io setflags noimmutable /mnt/f2fs/vdc.file
+get a flag on noimmutable ret=0, flags=800010
+set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable
+# rm /mnt/f2fs/vdc.file
+# df -h
+/dev/vdb                            64G  753M   64G   2% /mnt/f2fs
+
+So, the key idea is, user can do any file operations on /dev/vdc, and
+reclaim the space after the use, while the space is counted as /data.
+That doesn't require modifying partition size and filesystem format.