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f2fs: annotate lockless NAT counter reads

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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@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
This commit is contained in:
Cen Zhang 2026-05-05 20:55:10 +08:00 committed by Jaegeuk Kim
parent b952837f73
commit fb645a976f
2 changed files with 11 additions and 3 deletions
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6
fs/f2fs/node.c
View File

@ -73,7 +73,11 @@ 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))

8
fs/f2fs/node.h
View File

@ -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 {