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

[Yunlong Song]

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);
+		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