ext4: specify the free pointer offset for ext4_inode_cache

Convert ext4_inode_cache to use the kmem_cache_args interface and
specify a free pointer offset.

Since ext4_inode_cache uses a constructor, the free pointer would be
placed after the object to prevent overwriting fields used by the
constructor.

However, some fields such as ->i_flags are not used by the constructor
and can safely be repurposed for the free pointer.

Specify the free pointer offset at i_flags to reduce the object size.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260113061845.159790-4-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: hygoni

fs/ext4/super.c

@@ -1491,12 +1491,19 @@ static void init_once(void *foo)
 
 static int __init init_inodecache(void)
 {
-	ext4_inode_cachep = kmem_cache_create_usercopy("ext4_inode_cache",
-				sizeof(struct ext4_inode_info), 0,
-				SLAB_RECLAIM_ACCOUNT | SLAB_ACCOUNT,
-				offsetof(struct ext4_inode_info, i_data),
-				sizeof_field(struct ext4_inode_info, i_data),
-				init_once);
+	struct kmem_cache_args args = {
+		.useroffset = offsetof(struct ext4_inode_info, i_data),
+		.usersize = sizeof_field(struct ext4_inode_info, i_data),
+		.use_freeptr_offset = true,
+		.freeptr_offset = offsetof(struct ext4_inode_info, i_flags),
+		.ctor = init_once,
+	};
+
+	ext4_inode_cachep = kmem_cache_create("ext4_inode_cache",
+				sizeof(struct ext4_inode_info),
+				&args,
+				SLAB_RECLAIM_ACCOUNT | SLAB_ACCOUNT);
+
 	if (ext4_inode_cachep == NULL)
 		return -ENOMEM;
 	return 0;