Re: [PATCH v2] f2fs: prevent the current section from being selected as a victim during GC

[Chao Yu <chao@kernel.org>]

On 4/2/25 08:52, yohan.joung wrote:
> When selecting a victim using next_victim_seg in a large section, the
> selected section might already have been cleared and designated as the
> new current section, making it actively in use.
> This behavior causes inconsistency between the SIT and SSA.
> 
> F2FS-fs (dm-54): Inconsistent segment (70961) type [0, 1] in SSA and SIT
> Call trace:
> dump_backtrace+0xe8/0x10c
> show_stack+0x18/0x28
> dump_stack_lvl+0x50/0x6c
> dump_stack+0x18/0x28
> f2fs_stop_checkpoint+0x1c/0x3c
> do_garbage_collect+0x41c/0x271c
> f2fs_gc+0x27c/0x828
> gc_thread_func+0x290/0x88c
> kthread+0x11c/0x164
> ret_from_fork+0x10/0x20
> 
> issue scenario
> segs_per_sec=2
> - seg#0 and seg#1 are all dirty
> - all valid blocks are removed in seg#1
> - gc select this sec and next_victim_seg=seg#0
> - migrate seg#0, next_victim_seg=seg#1
> - checkpoint -> sec(seg#0, seg#1)  becomes free
> - allocator assigns sec(seg#0, seg#1) to curseg
> - gc tries to migrate seg#1
> 
> Signed-off-by: yohan.joung <yohan.joung@sk.com>
> Signed-off-by: Chao Yu <chao@kernel.org>
> ---
>  fs/f2fs/segment.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
> index 0465dc00b349..14d18bcf3559 100644
> --- a/fs/f2fs/segment.h
> +++ b/fs/f2fs/segment.h
> @@ -460,6 +460,7 @@ static inline void __set_test_and_free(struct f2fs_sb_info *sbi,
>  		unsigned int segno, bool inmem)
>  {
>  	struct free_segmap_info *free_i = FREE_I(sbi);
> +	struct dirty_seglist_info *dirty_i = DIRTY_I(sbi);
>  	unsigned int secno = GET_SEC_FROM_SEG(sbi, segno);
>  	unsigned int start_segno = GET_SEG_FROM_SEC(sbi, secno);
>  	unsigned int next;
> @@ -476,6 +477,11 @@ static inline void __set_test_and_free(struct f2fs_sb_info *sbi,
>  		if (next >= start_segno + usable_segs) {
>  			if (test_and_clear_bit(secno, free_i->free_secmap))
>  				free_i->free_sections++;
> +
> +			if (test_and_clear_bit(secno, dirty_i->victim_secmap)) {
> +				sbi->next_victim_seg[BG_GC] = NULL_SEGNO;
> +				sbi->next_victim_seg[FG_GC] = NULL_SEGNO;

sbi->next_victim_seg[FG_GC] relies on sbi->cur_victim_sec?

If sbi->next_victim_seg[BG_GC] is not equal to secno, will we still need to
nullify sbi->next_victim_seg[BG_GC]?

We have cleared bit in victim_secmap after we tag a section as prefree, right?

- locate_dirty_segment
 - __locate_dirty_segment
 - __remove_dirty_segment
  - clear_bit(GET_SEC_FROM_SEG(sbi, segno), dirty_i->victim_secmap);

Thanks,

> +			}
>  		}
>  	}
>  skip_free: