[PATCH v1 03/12] f2fs: cache: introduce shrinker

[Chao Yu]

This patch integrates the metadata cache into the F2FS memory shrinker
subsystem to reclaim clean, unreferenced cached blocks under memory
pressure.

It implements f2fs_shrink_cache() using a 3-phase cache reclamin method:
1. isolate clean entries from lru list
2. truncate from radix tree under lock
3. splice un-reclaimed entries back

And hooks the new interface into f2fs_shrink_count() and f2fs_shrink_scan().

Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
---
 fs/f2fs/cache.c    | 82 ++++++++++++++++++++++++++++++++++++++++++++++
 fs/f2fs/cache.h    |  3 ++
 fs/f2fs/shrinker.c | 12 +++++++
 3 files changed, 97 insertions(+)

diff --git a/fs/f2fs/cache.c b/fs/f2fs/cache.c
index 08bc658166f7..c071364822c4 100644
--- a/fs/f2fs/cache.c
+++ b/fs/f2fs/cache.c
@@ -529,3 +529,85 @@ void f2fs_destroy_cache(struct f2fs_cached_block_list *cache)
 	f2fs_put_cache(entry, true);
 	goto next;
 }
+
+static unsigned long f2fs_do_shrink_cache(struct f2fs_cached_block_list *cache,
+						unsigned long nr_to_scan)
+{
+	struct f2fs_cached_block *entry, *next;
+	LIST_HEAD(dispose_list);
+	LIST_HEAD(keep_list);
+	unsigned long freed = 0;
+	unsigned long isolated = 0;
+
+	/* Phase 1: Isolate candidate entries from LRU list into dispose_list */
+	spin_lock(&cache->list_lock);
+	list_for_each_entry_safe(entry, next, &cache->lru_list, list) {
+		if (isolated >= nr_to_scan)
+			break;
+
+		if (f2fs_cache_test_dirty(entry) ||
+		    f2fs_cache_test_writeback(entry) ||
+		    f2fs_cache_test_locked(entry))
+			continue;
+
+		if (f2fs_cache_refcount(entry) != 1)
+			continue;
+
+		list_move_tail(&entry->list, &dispose_list);
+		isolated++;
+	}
+	spin_unlock(&cache->list_lock);
+
+	/* Phase 2: Process isolated candidates one by one */
+	while (1) {
+		spin_lock(&cache->list_lock);
+		entry = list_first_entry_or_null(&dispose_list,
+						struct f2fs_cached_block, list);
+		if (!entry) {
+			spin_unlock(&cache->list_lock);
+			break;
+		}
+		f2fs_cache_get(entry);
+		list_move_tail(&entry->list, &keep_list);
+		spin_unlock(&cache->list_lock);
+
+		if (!f2fs_trylock_cache(entry)) {
+			f2fs_put_cache(entry, false);
+			continue;
+		}
+
+		/* the entry has been truncated */
+		if (!entry->cache) {
+			f2fs_put_cache(entry, true);
+			continue;
+		}
+		/*
+		 * at least there are shrinker, radix tree and another user
+		 * has referenced the entry.
+		 */
+		if (f2fs_cache_refcount(entry) >= 3) {
+			f2fs_put_cache(entry, true);
+			continue;
+		}
+
+		f2fs_do_truncate_cache(entry, false);
+
+		if (f2fs_put_cache(entry, true))
+			freed++;
+	}
+
+	/* Phase 3: Splice un-reclaimed entries back onto cache->lru_list */
+	if (!list_empty(&keep_list)) {
+		spin_lock(&cache->list_lock);
+		list_splice_tail(&keep_list, &cache->lru_list);
+		spin_unlock(&cache->list_lock);
+	}
+
+	return freed;
+}
+
+unsigned long f2fs_shrink_cache(struct f2fs_sb_info *sbi,
+					unsigned long nr_to_scan)
+{
+	return f2fs_do_shrink_cache(META_CACHE(sbi), nr_to_scan);
+}
diff --git a/fs/f2fs/cache.h b/fs/f2fs/cache.h
index 3cd2abafe364..d92b6f3ed585 100644
--- a/fs/f2fs/cache.h
+++ b/fs/f2fs/cache.h
@@ -188,4 +188,7 @@ void f2fs_stop_cache_wb_thread(struct f2fs_sb_info *sbi);
 #define f2fs_truncate_meta_caches(sbi, start, len)	\
 	f2fs_drop_cache_range(META_CACHE(sbi), start, len, true)
 
+unsigned long f2fs_shrink_cache(struct f2fs_sb_info *sbi,
+				unsigned long nr_to_scan);
+
 #endif /* _LINUX_F2FS_CACHE_H */
diff --git a/fs/f2fs/shrinker.c b/fs/f2fs/shrinker.c
index 4f6bf5926de4..1755c85849e4 100644
--- a/fs/f2fs/shrinker.c
+++ b/fs/f2fs/shrinker.c
@@ -37,6 +37,11 @@ static unsigned long __count_extent_cache(struct f2fs_sb_info *sbi,
 				atomic_read(&eti->total_ext_node);
 }
 
+static unsigned long __count_cache(struct f2fs_sb_info *sbi)
+{
+	return sbi->meta_blocks.num_entries;
+}
+
 unsigned long f2fs_shrink_count(struct shrinker *shrink,
 				struct shrink_control *sc)
 {
@@ -68,6 +73,9 @@ unsigned long f2fs_shrink_count(struct shrinker *shrink,
 		/* count free nids cache entries */
 		count += __count_free_nids(sbi);
 
+		/* count generic cache entries */
+		count += __count_cache(sbi);
+
 		spin_lock(&f2fs_list_lock);
 		p = p->next;
 		mutex_unlock(&sbi->umount_mutex);
@@ -120,6 +128,10 @@ unsigned long f2fs_shrink_scan(struct shrinker *shrink,
 		if (freed < nr)
 			freed += f2fs_try_to_free_nids(sbi, nr - freed);
 
+		/* shrink generic cache entries */
+		if (freed < nr)
+			freed += f2fs_shrink_cache(sbi, nr - freed);
+
 		spin_lock(&f2fs_list_lock);
 		p = p->next;
 		list_move_tail(&sbi->s_list, &f2fs_list);
-- 
2.49.0