Re: [f2fs-dev] [PATCH v4] f2fs: introduce fragment allocation mode mount option

[Chao Yu <chao@kernel.org>]

On 2021/9/4 4:33, Daeho Jeong wrote:
>>          if (f2fs_need_seq_seg(sbi))
>>                  return 0;
>>
>> static inline bool f2fs_need_seq_seg(struct f2fs_sb_info *sbi)
>> {
>>          return F2FS_OPTION(sbi).fs_mode == FS_MODE_FRAGMENT_FIXED_BLK;
>> }
>>
> 
> Do you need this in select_policy(), either?

IMO, for this mode, I prefer to disable background GC and just use newly
written userdata to fragment image, so I think it's fine to keep it as it is
here.

> Like,
>          if (f2fs_need_rand_seg(sbi))
>                  p->offset = prandom_u32() % (MAIN_SECS(sbi) *
> sbi->segs_per_sec);
>          else if (f2fs_need_seq_seg(sbi))
>                  p->offset = 0;
> 
>> One more concern... we'd better to save fragment_remained_hole as well
>> as fragment_remained_chunk,  otherwise, if fragment_chunk_size +
>> fragment_hole_size > 512, fragment hole will be truncated to 512 -
>> fragment_chunk_size due to we won't create hole with enough size as
>> seg->next_blkoff has crossed end of current segment.
>>
> 
> Sorry, I don't get it. You mean making fragment_remained_hole as a
> global variable?

As a per curseg field.

> Maybe, we run into the same race condition issue you told before for
> fragment_remained_chunk.
> Could you clarify this more?

e.g.

F2FS_OPTION(sbi).fs_mode = FS_MODE_FRAGMENT_FIXED_BLK
fragment_chunk_size = 384
fragment_hole_size = 384

When creating hole:

- f2fs_allocate_data_block
  - __refresh_next_blkoff
    chunk locates in [0, 383] of current segment
    seg->next_blkoff = 384
    sbi->fragment_remained_chunk = 0
    then we will reset sbi->fragment_remained_chunk to 384
    and move seg->next_blkoff forward to 768 (384 + 384)
  - __has_curseg_space() returns false
  - allocate_segment() allocates new current segment

So, for such case that hole may cross two segments, hole size may be truncated
to left size of previous segment.

Thanks,

> 
> Thanks,
>