Re: [PATCH v3] f2fs: fix missing discard for active segments

[Chao Yu]

On 3/17/25 18:16, Chunhai Guo wrote:
> 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@xxxxxxxx>

Reviewed-by: Chao Yu <chao@xxxxxxxxxx>

Thanks,