[PATCH] f2fs: annotate lockless NAT counter reads

[Cen Zhang]

nat_cnt[] is updated while callers hold nat_tree_lock, but F2FS samples
the counters locklessly in f2fs_available_free_memory(),
excess_dirty_nats(), and excess_cached_nats(). Those helpers only steer
cache reclaim and background sync heuristics; they do not control NAT
entry lifetime or checkpoint correctness.

Document the intent with data_race(READ_ONCE()) and a short comment
instead of adding locking to the balance path.

Signed-off-by: Cen Zhang <zzzccc427@xxxxxxxxx>
---
 fs/f2fs/node.c | 6 +++++-
 fs/f2fs/node.h | 8 ++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 74992fd9c9b6..8318c747600d 100644
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -72,7 +72,11 @@ bool f2fs_available_free_memory(struct f2fs_sb_info *sbi, int type)
 				sizeof(struct free_nid)) >> PAGE_SHIFT;
 		res = mem_size < ((avail_ram * nm_i->ram_thresh / 100) >> 2);
 	} else if (type == NAT_ENTRIES) {
-		mem_size = (nm_i->nat_cnt[TOTAL_NAT] *
+		/*
+		 * nat_cnt[] is heuristic accounting. Sample it locklessly here
+		 * to avoid taking nat_tree_lock in the balance path.
+		 */
+		mem_size = (data_race(READ_ONCE(nm_i->nat_cnt[TOTAL_NAT])) *
 				sizeof(struct nat_entry)) >> PAGE_SHIFT;
 		res = mem_size < ((avail_ram * nm_i->ram_thresh / 100) >> 2);
 		if (excess_cached_nats(sbi))
diff --git a/fs/f2fs/node.h b/fs/f2fs/node.h
index 824ac9f0e6e4..c1aa41b31d66 100644
--- a/fs/f2fs/node.h
+++ b/fs/f2fs/node.h
@@ -129,13 +129,17 @@ static inline void raw_nat_from_node_info(struct f2fs_nat_entry *raw_ne,
 
 static inline bool excess_dirty_nats(struct f2fs_sb_info *sbi)
 {
-	return NM_I(sbi)->nat_cnt[DIRTY_NAT] >= NM_I(sbi)->max_nid *
+	/* nat_cnt[] is heuristic accounting sampled locklessly here. */
+	return data_race(READ_ONCE(NM_I(sbi)->nat_cnt[DIRTY_NAT])) >=
+					NM_I(sbi)->max_nid *
 					NM_I(sbi)->dirty_nats_ratio / 100;
 }
 
 static inline bool excess_cached_nats(struct f2fs_sb_info *sbi)
 {
-	return NM_I(sbi)->nat_cnt[TOTAL_NAT] >= DEF_NAT_CACHE_THRESHOLD;
+	/* nat_cnt[] is heuristic accounting sampled locklessly here. */
+	return data_race(READ_ONCE(NM_I(sbi)->nat_cnt[TOTAL_NAT])) >=
+					DEF_NAT_CACHE_THRESHOLD;
 }
 
 enum mem_type {
-- 
2.43.0