f2fs: optimize NAT block loading during checkpoint write

Under stress tests with frequent metadata operations, checkpoint write
time can become excessively long. Analysis shows that the slowdown is
caused by synchronous, one-by-one reads of NAT blocks during checkpoint
processing.

The issue can be reproduced with the following workload:
1. seq 1 650000 | xargs -P 16 -n 1 touch
2. sync # avoid checkpoint write during deleting
3. delete 1 file every 455 files
4. echo 3 > /proc/sys/vm/drop_caches
5. sync # trigger checkpoint write

This patch submits read I/O for all NAT blocks required in the
__flush_nat_entry_set() phase in advance, reducing the overhead of
synchronous waiting for individual NAT block reads.

The NAT block flush latency before and after the change is as below:

|             |NAT blocks accessed|NAT blocks read|Flush time (ms)|
|-------------|-------------------|---------------|---------------|
|Before change|1205               |1191           |158            |
|After change |1264               |1242           |11             |

With a similar number of NAT blocks accessed and read from disk, adding
NAT block readahead reduces the total NAT block flush time by more than
90%.

Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: Yongpeng Yang

fs/f2fs/node.c

@@ -3179,7 +3179,7 @@ int f2fs_flush_nat_entries(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	struct f2fs_journal *journal = curseg->journal;
 	struct nat_entry_set *setvec[NAT_VEC_SIZE];
 	struct nat_entry_set *set, *tmp;
-	unsigned int found;
+	unsigned int found, entry_count = 0;
 	nid_t set_idx = 0;
 	LIST_HEAD(sets);
 	int err = 0;
@@ -3219,6 +3219,18 @@ int f2fs_flush_nat_entries(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 					MAX_NAT_JENTRIES(sbi, journal));
 	}
 
+	/*
+	 * Readahead the current NAT block to prevent read requests from
+	 * being issued and waited on one by one.
+	 */
+	list_for_each_entry(set, &sets, set_list) {
+		entry_count += set->entry_cnt;
+		if (!enabled_nat_bits(sbi, cpc) &&
+			__has_cursum_space(sbi, journal,
+					entry_count, NAT_JOURNAL))
+			continue;
+		f2fs_ra_meta_pages(sbi, set->set, 1, META_NAT, true);
+	}
 	/* flush dirty nats in nat entry set */
 	list_for_each_entry_safe(set, tmp, &sets, set_list) {
 		err = __flush_nat_entry_set(sbi, set, cpc);