f2fs: fix missing discard for active segments

f2fs: fix missing discard for active segments

During a checkpoint, the current active segment X may not be handled
properly. This occurs when segment X has 0 valid blocks and a non-zero
number of discard blocks, for the following reasons:

locate_dirty_segment() does not mark any active segment as a prefree
segment. As a result, segment X is not included in dirty_segmap[PRE], and
f2fs_clear_prefree_segments() skips it when handling prefree segments.

add_discard_addrs() skips any segment with 0 valid blocks, so segment X is
also skipped.

Consequently, no `struct discard_cmd` is actually created for segment X.
However, the ckpt_valid_map and cur_valid_map of segment X are synced by
seg_info_to_raw_sit() during the current checkpoint process. As a result,
it cannot find the missing discard bits even in subsequent checkpoints.
Consequently, the value of sbi->discard_blks remains non-zero. Thus, when
f2fs is umounted, CP_TRIMMED_FLAG will not be set due to the non-zero
sbi->discard_blks.

Relevant code process:

f2fs_write_checkpoint()
    f2fs_flush_sit_entries()
         list_for_each_entry_safe(ses, tmp, head, set_list) {
             for_each_set_bit_from(segno, bitmap, end) {
                 ...
                 add_discard_addrs(sbi, cpc, false); // skip segment X due to its 0 valid blocks
                 ...
                 seg_info_to_raw_sit(); // sync ckpt_valid_map with cur_valid_map for segment X
                 ...
             }
         }
    f2fs_clear_prefree_segments(); // segment X is not included in dirty_segmap[PRE] and is skipped

This issue is easy to reproduce with the following operations:

root # mkfs.f2fs -f /dev/f2fs_dev
root # mount -t f2fs /dev/f2fs_dev /mnt_point
root # dd if=/dev/blk_dev of=/mnt_point/1.bin bs=4k count=256
root # sync
root # rm /mnt_point/1.bin
root # umount /mnt_point
root # dump.f2fs /dev/f2fs_dev | grep "checkpoint state"
Info: checkpoint state = 45 :  crc compacted_summary unmount ---- 'trimmed' flag is missing

Since add_discard_addrs() can handle active segments with non-zero valid
blocks, it is reasonable to fix this issue by allowing it to also handle
active segments with 0 valid blocks.

Fixes: b29555505d81 ("f2fs: add key functions for small discards")
Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>