f2fs: expose main_blkaddr in sysfs

Expose in /sys/fs/f2fs/<blockdev>/main_blkaddr the block address where the
main area starts. This allows user mode programs to determine:

- That pinned files that are made exclusively of fully allocated 2MB
  segments will never be unpinned by the file system.

- Where the main area starts. This is required by programs that want to
  verify if a file is made exclusively of 2MB f2fs segments, the alignment
  boundary for segments starts at this address. Testing for 2MB alignment
  relative to the start of the device is incorrect, because for some
  filesystems main_blkaddr is not at a 2MB boundary relative to the start
  of the device.

The entry will be used when validating reliable pinning file feature proposed
by "f2fs: support aligned pinned file".
Signed-off-by: Ramon Pantin <pantin@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -31,6 +31,12 @@ Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>
 Description:
 		 Controls the issue rate of segment discard commands.
 
+What:		/sys/fs/f2fs/<disk>/max_blkaddr
+Date:		November 2019
+Contact:	"Ramon Pantin" <pantin@google.com>
+Description:
+		 Shows first block address of MAIN area.
+
 What:		/sys/fs/f2fs/<disk>/ipu_policy
 Date:		November 2013
 Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>

Documentation/filesystems/f2fs.txt

@@ -297,6 +297,9 @@ Files in /sys/fs/f2fs/<devname>
 			      reclaim the prefree segments to free segments.
 			      By default, 5% over total # of segments.
 
+ main_blkaddr                 This value gives the first block address of
+			      MAIN area in the partition.
+
  max_small_discards	      This parameter controls the number of discard
 			      commands that consist small blocks less than 2MB.
 			      The candidates to be discarded are cached until

fs/f2fs/sysfs.c

@@ -445,6 +445,7 @@ F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_no_gc_sleep_time, no_gc_sleep_time);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, gc_idle, gc_mode);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, gc_urgent, gc_mode);
 F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, reclaim_segments, rec_prefree_segments);
+F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, main_blkaddr, main_blkaddr);
 F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, max_small_discards, max_discards);
 F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, discard_granularity, discard_granularity);
 F2FS_RW_ATTR(RESERVED_BLOCKS, f2fs_sb_info, reserved_blocks, reserved_blocks);
@@ -512,6 +513,7 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(gc_idle),
 	ATTR_LIST(gc_urgent),
 	ATTR_LIST(reclaim_segments),
+	ATTR_LIST(main_blkaddr),
 	ATTR_LIST(max_small_discards),
 	ATTR_LIST(discard_granularity),
 	ATTR_LIST(batched_trim_sections),