f2fs: reclaim prefree segments periodically

Previously, f2fs postpones reclaiming prefree segments into free segments
as much as possible.
However, if user writes and deletes a bunch of data without any sync or fsync
calls, some flash storages can suffer from garbage collections.

So, this patch adds the reclaiming codes to f2fs_write_node_pages and background
GC thread.

If there are a lot of prefree segments, let's do checkpoint so that f2fs
submits discard commands for the prefree regions to the flash storage.
Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/f2fs.h

@@ -299,6 +299,9 @@ struct f2fs_sm_info {
 	unsigned int main_segments;	/* # of segments in main area */
 	unsigned int reserved_segments;	/* # of reserved segments */
 	unsigned int ovp_segments;	/* # of overprovision segments */
+
+	/* a threshold to reclaim prefree segments */
+	unsigned int rec_prefree_segments;
 };
 
 /*

fs/f2fs/gc.c

@@ -82,6 +82,11 @@ static int gc_thread_func(void *data)
 		/* if return value is not zero, no victim was selected */
 		if (f2fs_gc(sbi))
 			wait_ms = gc_th->no_gc_sleep_time;
+
+		/* balancing prefree segments */
+		if (excess_prefree_segs(sbi))
+			f2fs_sync_fs(sbi->sb, true);
+
 	} while (!kthread_should_stop());
 	return 0;
 }

fs/f2fs/node.c

@@ -1206,7 +1206,8 @@ static int f2fs_write_node_pages(struct address_space *mapping,
 	long nr_to_write = wbc->nr_to_write;
 
 	/* First check balancing cached NAT entries */
-	if (try_to_free_nats(sbi, NAT_ENTRY_PER_BLOCK)) {
+	if (try_to_free_nats(sbi, NAT_ENTRY_PER_BLOCK) ||
+				excess_prefree_segs(sbi)) {
 		f2fs_sync_fs(sbi->sb, true);
 		return 0;
 	}

fs/f2fs/segment.c

@@ -1645,6 +1645,7 @@ int build_segment_manager(struct f2fs_sb_info *sbi)
 	sm_info->ovp_segments = le32_to_cpu(ckpt->overprov_segment_count);
 	sm_info->main_segments = le32_to_cpu(raw_super->segment_count_main);
 	sm_info->ssa_blkaddr = le32_to_cpu(raw_super->ssa_blkaddr);
+	sm_info->rec_prefree_segments = DEF_RECLAIM_PREFREE_SEGMENTS;
 
 	err = build_sit_info(sbi);
 	if (err)

fs/f2fs/segment.h

@@ -14,6 +14,8 @@
 #define NULL_SEGNO			((unsigned int)(~0))
 #define NULL_SECNO			((unsigned int)(~0))
 
+#define DEF_RECLAIM_PREFREE_SEGMENTS	100	/* 200MB of prefree segments */
+
 /* L: Logical segment # in volume, R: Relative segment # in main area */
 #define GET_L2R_SEGNO(free_i, segno)	(segno - free_i->start_segno)
 #define GET_R2L_SEGNO(free_i, segno)	(segno + free_i->start_segno)
@@ -472,6 +474,11 @@ static inline bool has_not_enough_free_secs(struct f2fs_sb_info *sbi, int freed)
 						reserved_sections(sbi)));
 }
 
+static inline bool excess_prefree_segs(struct f2fs_sb_info *sbi)
+{
+	return (prefree_segments(sbi) > SM_I(sbi)->rec_prefree_segments);
+}
+
 static inline int utilization(struct f2fs_sb_info *sbi)
 {
 	return div_u64((u64)valid_user_blocks(sbi) * 100, sbi->user_block_count);