f2fs: support resizable tail section and unify pinned allocation

Currently, zoned block devices restrict pinned file allocations to
conventional zones at the beginning of the storage (before
first_seq_zone_segno), triggering range GC when conventional space is
exhausted.

On regular block devices, when preparing for future online filesystem
resizing (e.g. partition shrinking), pinned files must not be allocated
in the tail area that will be truncated, as pinned files cannot be
relocated by GC. Specifying the resizable tail area size (in sections)
allows uniform mount configuration across devices of different storage
capacities.

To support this, introduce a unified `pinned_area_max_secno` boundary
abstraction in `f2fs_sb_info`:
1. Add `-o resizable_tail_secno=%u` mount option to specify the number
   of sections at the tail of the filesystem reserved for resizing.
2. In `f2fs_fill_super()`, initialize `sbi->pinned_area_max_secno` as:
   min(MAIN_SECS(sbi) - resizable_tail_sec, zoned_max_sec).
3. In `get_new_segment()`, restrict segment allocation for pinned files
   (`pinning == true`) to `0 .. sbi->pinned_area_max_secno - 1`. If no
   free section is available in the pinned area, return -EAGAIN.
4. In `f2fs_allocate_pinning_section()`, unify the range GC trigger to
   run `f2fs_gc_range()` up to `sbi->pinned_area_max_secno` whenever
   `sbi->pinned_area_max_secno < MAIN_SECS(sbi)` and allocation
   returns -EAGAIN.
5. Expose `/sys/fs/f2fs/<dev>/pinned_area_max_secno` as a read-only
   sysfs node.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Sunmin Jeong <s_min.jeong@samsung.com>
Reviewed-by: Wenjie Qi <qiwenjie@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

2 files changed, 14 insertions, 1 deletions

diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
index 85194e4c7f01..0cebc89799dd 100644
--- a/Documentation/ABI/testing/sysfs-fs-f2fs
+++ b/Documentation/ABI/testing/sysfs-fs-f2fs
@@ -1013,3 +1013,10 @@ Description:	Every time a write operation completes f2fs_write_end_io() is
 		the maximum size of a write bio that is completed in atomic
 		(atc) context. The default value for this attribute is UINT_MAX
 		which means that this functionality is disabled by default.
+
+What:		/sys/fs/f2fs/<disk>/pinned_area_max_secno
+Date:		August 2026
+Contact:	"Daeho Jeong" <daehojeong@google.com>
+Description:	This is a read-only entry to show the upper bound section number
+		for pinned files. Pinned files will only be allocated within
+		sections 0 to pinned_area_max_secno - 1.
diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
index 1a5fd4afe609..a3c3b6948734 100644
--- a/Documentation/filesystems/f2fs.rst
+++ b/Documentation/filesystems/f2fs.rst
@@ -417,7 +417,13 @@ lookup_mode=%s		 Control the directory lookup behavior for casefolded
 			     auto               F2FS determines the mode based on the
 					        on-disk `SB_ENC_NO_COMPAT_FALLBACK_FL`
 					        flag.
-			     ================== ========================================
+resizable_tail_secno=%u  Control the number of sections at the tail of the
+			 filesystem reserved for online resizing. Pinned files
+			 will only be allocated within sections 0 to
+			 (MAIN_SECS - resizable_tail_secno) - 1. If set to 0
+			 (default), there is no tail restriction unless running
+			 on a zoned block device where conventional zones are
+			 used.
 ======================== ============================================================
 
 Debugfs Entries