f2fs: shrink spinlock coverage

In f2fs_try_to_free_nids(), .nid_list_lock spinlock critical region will
increase as expected shrink number increase, to avoid spining other CPUs
for long time, we change to release nid caches with small batch each time
under .nid_list_lock coverage.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -2488,7 +2488,6 @@ void f2fs_alloc_nid_failed(struct f2fs_sb_info *sbi, nid_t nid)
 int f2fs_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 (nm_i->nid_cnt[FREE_NID] <= MAX_FREE_NIDS)
@@ -2497,17 +2496,23 @@ int f2fs_try_to_free_nids(struct f2fs_sb_info *sbi, int nr_shrink)
 	if (!mutex_trylock(&nm_i->build_lock))
 		return 0;
 
-	spin_lock(&nm_i->nid_list_lock);
-	list_for_each_entry_safe(i, next, &nm_i->free_nid_list, list) {
-		if (nr_shrink <= 0 ||
-				nm_i->nid_cnt[FREE_NID] <= MAX_FREE_NIDS)
-			break;
+	while (nr_shrink && nm_i->nid_cnt[FREE_NID] > MAX_FREE_NIDS) {
+		struct free_nid *i, *next;
+		unsigned int batch = SHRINK_NID_BATCH_SIZE;
 
-		__remove_free_nid(sbi, i, FREE_NID);
-		kmem_cache_free(free_nid_slab, i);
-		nr_shrink--;
+		spin_lock(&nm_i->nid_list_lock);
+		list_for_each_entry_safe(i, next, &nm_i->free_nid_list, list) {
+			if (!nr_shrink || !batch ||
+				nm_i->nid_cnt[FREE_NID] <= MAX_FREE_NIDS)
+				break;
+			__remove_free_nid(sbi, i, FREE_NID);
+			kmem_cache_free(free_nid_slab, i);
+			nr_shrink--;
+			batch--;
+		}
+		spin_unlock(&nm_i->nid_list_lock);
 	}
-	spin_unlock(&nm_i->nid_list_lock);
+
 	mutex_unlock(&nm_i->build_lock);
 
 	return nr - nr_shrink;

fs/f2fs/node.h

@@ -15,6 +15,9 @@
 #define FREE_NID_PAGES	8
 #define MAX_FREE_NIDS	(NAT_ENTRY_PER_BLOCK * FREE_NID_PAGES)
 
+/* size of free nid batch when shrinking */
+#define SHRINK_NID_BATCH_SIZE	8
+
 #define DEF_RA_NID_PAGES	0	/* # of nid pages to be readaheaded */
 
 /* maximum readahead size for node during getting data blocks */