f2fs: show number of pending discard commands

This information can be used to check how much time we need to give to issue
all the discard commands.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -112,6 +112,11 @@ Contact:	"Jaegeuk Kim" <jaegeuk@kernel.org>
 Description:	Set timeout to issue discard commands during umount.
 	        Default: 5 secs
 
+What:		/sys/fs/f2fs/<disk>/pending_discard
+Date:		November 2021
+Contact:	"Jaegeuk Kim" <jaegeuk@kernel.org>
+Description:	Shows the number of pending discard commands in the queue.
+
 What:		/sys/fs/f2fs/<disk>/max_victim_search
 Date:		January 2014
 Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>

fs/f2fs/sysfs.c

@@ -118,6 +118,15 @@ static ssize_t sb_status_show(struct f2fs_attr *a,
 	return sprintf(buf, "%lx\n", sbi->s_flag);
 }
 
+static ssize_t pending_discard_show(struct f2fs_attr *a,
+		struct f2fs_sb_info *sbi, char *buf)
+{
+	if (!SM_I(sbi)->dcc_info)
+		return -EINVAL;
+	return sprintf(buf, "%llu\n", (unsigned long long)atomic_read(
+				&SM_I(sbi)->dcc_info->discard_cmd_cnt));
+}
+
 static ssize_t features_show(struct f2fs_attr *a,
 		struct f2fs_sb_info *sbi, char *buf)
 {
@@ -744,6 +753,7 @@ F2FS_GENERAL_RO_ATTR(unusable);
 F2FS_GENERAL_RO_ATTR(encoding);
 F2FS_GENERAL_RO_ATTR(mounted_time_sec);
 F2FS_GENERAL_RO_ATTR(main_blkaddr);
+F2FS_GENERAL_RO_ATTR(pending_discard);
 #ifdef CONFIG_F2FS_STAT_FS
 F2FS_STAT_ATTR(STAT_INFO, f2fs_stat_info, cp_foreground_calls, cp_count);
 F2FS_STAT_ATTR(STAT_INFO, f2fs_stat_info, cp_background_calls, bg_cp_count);
@@ -812,6 +822,7 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(main_blkaddr),
 	ATTR_LIST(max_small_discards),
 	ATTR_LIST(discard_granularity),
+	ATTR_LIST(pending_discard),
 	ATTR_LIST(batched_trim_sections),
 	ATTR_LIST(ipu_policy),
 	ATTR_LIST(min_ipu_util),