f2fs: shrink free_nids entries

This patch introduces __count_free_nids/try_to_free_nids and registers
them in slab shrinker for shrinking under memory pressure.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -1635,6 +1635,34 @@ void alloc_nid_failed(struct f2fs_sb_info *sbi, nid_t nid)
 		kmem_cache_free(free_nid_slab, i);
 }
 
+int try_to_free_nids(struct f2fs_sb_info *sbi, int nr_shrink)
+{
+	struct f2fs_nm_info *nm_i = NM_I(sbi);
+	struct free_nid *i, *next;
+	int nr = nr_shrink;
+
+	if (!mutex_trylock(&nm_i->build_lock))
+		return 0;
+
+	spin_lock(&nm_i->free_nid_list_lock);
+	list_for_each_entry_safe(i, next, &nm_i->free_nid_list, list) {
+		if (nr_shrink <= 0 || nm_i->fcnt <= NAT_ENTRY_PER_BLOCK)
+			break;
+		if (i->state == NID_ALLOC)
+			continue;
+		__del_from_free_nid_list(nm_i, i);
+		nm_i->fcnt--;
+		spin_unlock(&nm_i->free_nid_list_lock);
+		kmem_cache_free(free_nid_slab, i);
+		nr_shrink--;
+		spin_lock(&nm_i->free_nid_list_lock);
+	}
+	spin_unlock(&nm_i->free_nid_list_lock);
+	mutex_unlock(&nm_i->build_lock);
+
+	return nr - nr_shrink;
+}
+
 void recover_inline_xattr(struct inode *inode, struct page *page)
 {
 	void *src_addr, *dst_addr;