Re: [PATCH RFC] f2fs: fix to avoid selecting full segment w/ {AT,}SSR allocator

[Chao Yu]

Jaegeuk,

Could you please help to review this patch? since I doubt that this
issue can happen in real world... :(

Thanks,

On 2021/2/22 21:43, Chao Yu wrote:
> Ping,
>
> On 2021/2/20 17:40, Chao Yu wrote:
> > In cp disabling mode, there could be a condition
> > - target segment has 128 ckpt valid blocks
> > - GC migrates 128 valid blocks to other segment (segment is still in
> > dirty list)
> > - GC migrates 384 blocks to target segment (segment has 128 cp_vblocks
> > and 384 vblocks)
> > - If GC selects target segment via {AT,}SSR allocator, however there is
> > no free space in targe segment.
> >
> > Fixes: 4354994f097d ("f2fs: checkpoint disabling")
> > Fixes: 093749e296e2 ("f2fs: support age threshold based garbage collection")
> > Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
> > ---
> >    fs/f2fs/f2fs.h    |  1 +
> >    fs/f2fs/gc.c      | 17 +++++++++++++----
> >    fs/f2fs/segment.c | 20 ++++++++++++++++++++
> >    3 files changed, 34 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index ed7807103c8e..9c753eff0814 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -3376,6 +3376,7 @@ block_t f2fs_get_unusable_blocks(struct f2fs_sb_info *sbi);
> >    int f2fs_disable_cp_again(struct f2fs_sb_info *sbi, block_t unusable);
> >    void f2fs_release_discard_addrs(struct f2fs_sb_info *sbi);
> >    int f2fs_npages_for_summary_flush(struct f2fs_sb_info *sbi, bool for_ra);
> > +bool segment_has_free_slot(struct f2fs_sb_info *sbi, int segno);
> >    void f2fs_init_inmem_curseg(struct f2fs_sb_info *sbi);
> >    void f2fs_save_inmem_curseg(struct f2fs_sb_info *sbi);
> >    void f2fs_restore_inmem_curseg(struct f2fs_sb_info *sbi);
> > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > index 86ba8ed0b8a7..a1d8062cdace 100644
> > --- a/fs/f2fs/gc.c
> > +++ b/fs/f2fs/gc.c
> > @@ -392,10 +392,6 @@ static void add_victim_entry(struct f2fs_sb_info *sbi,
> >    		if (p->gc_mode == GC_AT &&
> >    			get_valid_blocks(sbi, segno, true) == 0)
> >    			return;
> > -
> > -		if (p->alloc_mode == AT_SSR &&
> > -			get_seg_entry(sbi, segno)->ckpt_valid_blocks == 0)
> > -			return;
> >    	}
> >    
> >    	for (i = 0; i < sbi->segs_per_sec; i++)
> > @@ -736,6 +732,19 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
> >    		if (gc_type == BG_GC && test_bit(secno, dirty_i->victim_secmap))
> >    			goto next;
> >    
> > +		if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED))) {
> > +			/*
> > +			 * to avoid selecting candidate which has below valid
> > +			 * block distribution:
> > +			 * partial blocks are valid and all left ones are valid
> > +			 * in previous checkpoint.
> > +			 */
> > +			if (p.alloc_mode == SSR || p.alloc_mode == AT_SSR) {
> > +				if (!segment_has_free_slot(sbi, segno))
> > +					goto next;
> > +			}
> > +		}
> > +
> >    		if (is_atgc) {
> >    			add_victim_entry(sbi, &p, segno);
> >    			goto next;
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index 2d5a82c4ca15..deaf57e13125 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -2650,6 +2650,26 @@ static void __refresh_next_blkoff(struct f2fs_sb_info *sbi,
> >    		seg->next_blkoff++;
> >    }
> >    
> > +bool segment_has_free_slot(struct f2fs_sb_info *sbi, int segno)
> > +{
> > +	struct sit_info *sit = SIT_I(sbi);
> > +	struct seg_entry *se = get_seg_entry(sbi, segno);
> > +	int entries = SIT_VBLOCK_MAP_SIZE / sizeof(unsigned long);
> > +	unsigned long *target_map = SIT_I(sbi)->tmp_map;
> > +	unsigned long *ckpt_map = (unsigned long *)se->ckpt_valid_map;
> > +	unsigned long *cur_map = (unsigned long *)se->cur_valid_map;
> > +	int i, pos;
> > +
> > +	down_write(&sit->sentry_lock);
> > +	for (i = 0; i < entries; i++)
> > +		target_map[i] = ckpt_map[i] | cur_map[i];
> > +
> > +	pos = __find_rev_next_zero_bit(target_map, sbi->blocks_per_seg, 0);
> > +	up_write(&sit->sentry_lock);
> > +
> > +	return pos < sbi->blocks_per_seg;
> > +}
> > +
> >    /*
> >     * This function always allocates a used segment(from dirty seglist) by SSR
> >     * manner, so it should recover the existing segment information of valid blocks
> .