Re: [PATCH] f2fs: let __get_victim successfully get a segno in corner case

[Yunlong Song <yunlong.song@xxxxxxxxxx>]

Hi, Jay,

Sorry for the mistake in last mail, the ovp is 462, and the reserved is 235.
I check the code and have not found problems with p.max_search yet.

Just forget the this patch, since there is still 870 segments below, so 
it should
not be the assumed case of this patch.

By the way, I have sent another patch to provide f2fs_balance_fs to 
__write_node_page,
similar as that in __write_data_page, to avoid missing f2fs_balance_fs 
in node page writeback.
Please check that.

On 2017/7/26 2:55, Jaegeuk Kim wrote:
> On 07/14, Yunlong Song wrote:
> > Suppose that the valid blocks of each section are all over sbi->fggc_threshold,
> > and even has_not_enough_free_secs is true, f2fs_gc cannot do its job since the
> > no_fggc_candidate always returns true. As a result, the reserved segments can be
> > used up, and finally there is no free segment at all, and get_new_segment cannot
> > get a free segment, filesystem will trap into a wrong status.
> >
> > To fix this, we record the segno which has a rough minimum cost and return it to
> > __get_victim to continue f2fs_gc's job.
> >
> > Signed-off-by: Yunlong Song <yunlong.song@xxxxxxxxxx>
> > ---
> >   fs/f2fs/gc.c      | 19 ++++++++++++++-----
> >   fs/f2fs/segment.h | 17 ++++++++++++++---
> >   2 files changed, 28 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > index fa3d2e2..965e783 100644
> > --- a/fs/f2fs/gc.c
> > +++ b/fs/f2fs/gc.c
> > @@ -178,6 +178,8 @@ static void select_policy(struct f2fs_sb_info *sbi, int gc_type,
> >   		p->offset = 0;
> >   	else
> >   		p->offset = SIT_I(sbi)->last_victim[p->gc_mode];
> > +
> > +	p->min_cost_r = UINT_MAX;
> >   }
> >   
> >   static unsigned int get_max_cost(struct f2fs_sb_info *sbi,
> > @@ -194,7 +196,7 @@ static unsigned int get_max_cost(struct f2fs_sb_info *sbi,
> >   		return 0;
> >   }
> >   
> > -static unsigned int check_bg_victims(struct f2fs_sb_info *sbi)
> > +static unsigned int check_bg_victims(struct f2fs_sb_info *sbi, struct victim_sel_policy *p)
> >   {
> >   	struct dirty_seglist_info *dirty_i = DIRTY_I(sbi);
> >   	unsigned int secno;
> > @@ -208,11 +210,12 @@ static unsigned int check_bg_victims(struct f2fs_sb_info *sbi)
> >   		if (sec_usage_check(sbi, secno))
> >   			continue;
> >   
> > -		if (no_fggc_candidate(sbi, secno))
> > +		p->cur_segno_r = GET_SEG_FROM_SEC(sbi, secno);
> > +		if (no_fggc_candidate(sbi, secno, p))
> >   			continue;
> >   
> >   		clear_bit(secno, dirty_i->victim_secmap);
> > -		return GET_SEG_FROM_SEC(sbi, secno);
> > +		return p->cur_segno_r;
> >   	}
> >   	return NULL_SEGNO;
> >   }
> > @@ -332,7 +335,7 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
> >   
> >   	last_victim = sm->last_victim[p.gc_mode];
> >   	if (p.alloc_mode == LFS && gc_type == FG_GC) {
> > -		p.min_segno = check_bg_victims(sbi);
> > +		p.min_segno = check_bg_victims(sbi, &p);
> >   		if (p.min_segno != NULL_SEGNO)
> >   			goto got_it;
> >   	}
> > @@ -369,8 +372,9 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
> >   			goto next;
> >   		if (gc_type == BG_GC && test_bit(secno, dirty_i->victim_secmap))
> >   			goto next;
> > +		p.cur_segno_r = segno;
> >   		if (gc_type == FG_GC && p.alloc_mode == LFS &&
> > -					no_fggc_candidate(sbi, secno))
> > +					no_fggc_candidate(sbi, secno, &p))
> >   			goto next;
> >   
> >   		cost = get_gc_cost(sbi, segno, &p);
> > @@ -403,6 +407,11 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
> >   		trace_f2fs_get_victim(sbi->sb, type, gc_type, &p,
> >   				sbi->cur_victim_sec,
> >   				prefree_segments(sbi), free_segments(sbi));
> > +	} else if (has_not_enough_free_secs(sbi, 0, 0)) {
> > +		p.min_segno = p.min_segno_r;
> > +		if (p.alloc_mode == LFS && gc_type == FG_GC)
> > +			clear_bit(GET_SEC_FROM_SEG(sbi, p.min_segno), dirty_i->victim_secmap);
> It seems you want to give a victim segment which has valid blocks larger than
> fggc_threshold.
>
> fggc: 507
> reserve: 462
> cnt_full: 25182
> over: 25912 (>= fggc)
> less: 870 (< fggc)
>
> What is ovp segments?
> It looks like there are 870 candidates for FG_GC, and GC with 103 segments must
> give 104 free segments in the worst case. Is there a problem on p.max_search?
>
> Thanks,
>
> > +		goto got_it;
> >   	}
> >   out:
> >   	mutex_unlock(&dirty_i->seglist_lock);
> > diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
> > index 6b871b4..7d2d0f3 100644
> > --- a/fs/f2fs/segment.h
> > +++ b/fs/f2fs/segment.h
> > @@ -169,6 +169,9 @@ struct victim_sel_policy {
> >   	unsigned int ofs_unit;		/* bitmap search unit */
> >   	unsigned int min_cost;		/* minimum cost */
> >   	unsigned int min_segno;		/* segment # having min. cost */
> > +	unsigned int min_cost_r;		/* rough minimum cost */
> > +	unsigned int min_segno_r;		/* segment # having rough min. cost */
> > +	unsigned int cur_segno_r;		/* segment # rough process is handling */
> >   };
> >   
> >   struct seg_entry {
> > @@ -743,11 +746,19 @@ static inline block_t sum_blk_addr(struct f2fs_sb_info *sbi, int base, int type)
> >   }
> >   
> >   static inline bool no_fggc_candidate(struct f2fs_sb_info *sbi,
> > -						unsigned int secno)
> > +						unsigned int secno, struct victim_sel_policy *p)
> >   {
> > -	if (get_valid_blocks(sbi, GET_SEG_FROM_SEC(sbi, secno), true) >=
> > -						sbi->fggc_threshold)
> > +	unsigned int cur_cost;
> > +
> > +	cur_cost = get_valid_blocks(sbi, GET_SEG_FROM_SEC(sbi, secno), true);
> > +	if (cur_cost >= sbi->fggc_threshold) {
> > +		if (p->min_cost_r > cur_cost) {
> > +			p->min_cost_r = cur_cost;
> > +			p->min_segno_r = p->cur_segno_r;
> > +		}
> >   		return true;
> > +	}
> > +
> >   	return false;
> >   }
> >   
> > --
> > 1.8.5.2
> .

--
Thanks,
Yunlong Song