f2fs: export migration_granularity sysfs entry

Add one sysfs entry to control migration granularity of GC in large
section f2fs, it can be tuned to mitigate heavy overhead of migrating
huge number of blocks in large section.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -92,6 +92,15 @@ Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>
 Description:
 		 Controls the number of trials to find a victim segment.
 
+What:		/sys/fs/f2fs/<disk>/migration_granularity
+Date:		October 2018
+Contact:	"Chao Yu" <yuchao0@huawei.com>
+Description:
+		 Controls migration granularity of garbage collection on large
+		 section, it can let GC move partial segment{s} of one section
+		 in one GC cycle, so that dispersing heavy overhead GC to
+		 multiple lightweight one.
+
 What:		/sys/fs/f2fs/<disk>/dir_level
 Date:		March 2014
 Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>

fs/f2fs/sysfs.c

@@ -246,6 +246,11 @@ static ssize_t __sbi_store(struct f2fs_attr *a,
 		return count;
 	}
 
+	if (!strcmp(a->attr.name, "migration_granularity")) {
+		if (t == 0 || t > sbi->segs_per_sec)
+			return -EINVAL;
+	}
+
 	if (!strcmp(a->attr.name, "trim_sections"))
 		return -EINVAL;
 
@@ -406,6 +411,7 @@ F2FS_RW_ATTR(NM_INFO, f2fs_nm_info, ram_thresh, ram_thresh);
 F2FS_RW_ATTR(NM_INFO, f2fs_nm_info, ra_nid_pages, ra_nid_pages);
 F2FS_RW_ATTR(NM_INFO, f2fs_nm_info, dirty_nats_ratio, dirty_nats_ratio);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, max_victim_search, max_victim_search);
+F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, migration_granularity, migration_granularity);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, dir_level, dir_level);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, cp_interval, interval_time[CP_TIME]);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, idle_interval, interval_time[REQ_TIME]);
@@ -460,6 +466,7 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(min_hot_blocks),
 	ATTR_LIST(min_ssr_sections),
 	ATTR_LIST(max_victim_search),
+	ATTR_LIST(migration_granularity),
 	ATTR_LIST(dir_level),
 	ATTR_LIST(ram_thresh),
 	ATTR_LIST(ra_nid_pages),