btrfs: remove hole from struct btrfs_delayed_node

On x86_64 and a release kernel, there's a 4 bytes hole in the structure
after the ref count field:

  struct btrfs_delayed_node {
          u64                        inode_id;             /*     0     8 */
          u64                        bytes_reserved;       /*     8     8 */
          struct btrfs_root *        root;                 /*    16     8 */
          struct list_head           n_list;               /*    24    16 */
          struct list_head           p_list;               /*    40    16 */
          struct rb_root_cached      ins_root;             /*    56    16 */
          /* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
          struct rb_root_cached      del_root;             /*    72    16 */
          struct mutex               mutex;                /*    88    32 */
          struct btrfs_inode_item    inode_item;           /*   120   160 */
          /* --- cacheline 4 boundary (256 bytes) was 24 bytes ago --- */
          refcount_t                 refs;                 /*   280     4 */

          /* XXX 4 bytes hole, try to pack */

          u64                        index_cnt;            /*   288     8 */
          long unsigned int          flags;                /*   296     8 */
          int                        count;                /*   304     4 */
          u32                        curr_index_batch_size; /*   308     4 */
          u32                        index_item_leaves;    /*   312     4 */

          /* size: 320, cachelines: 5, members: 15 */
          /* sum members: 312, holes: 1, sum holes: 4 */
          /* padding: 4 */
  };

Move the 'count' field, which is 4 bytes long, to just below the ref count
field, so we eliminate the hole and reduce the structure size from 320
bytes down to 312 bytes:

  struct btrfs_delayed_node {
          u64                        inode_id;             /*     0     8 */
          u64                        bytes_reserved;       /*     8     8 */
          struct btrfs_root *        root;                 /*    16     8 */
          struct list_head           n_list;               /*    24    16 */
          struct list_head           p_list;               /*    40    16 */
          struct rb_root_cached      ins_root;             /*    56    16 */
          /* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
          struct rb_root_cached      del_root;             /*    72    16 */
          struct mutex               mutex;                /*    88    32 */
          struct btrfs_inode_item    inode_item;           /*   120   160 */
          /* --- cacheline 4 boundary (256 bytes) was 24 bytes ago --- */
          refcount_t                 refs;                 /*   280     4 */
          int                        count;                /*   284     4 */
          u64                        index_cnt;            /*   288     8 */
          long unsigned int          flags;                /*   296     8 */
          u32                        curr_index_batch_size; /*   304     4 */
          u32                        index_item_leaves;    /*   308     4 */

          /* size: 312, cachelines: 5, members: 15 */
          /* last cacheline: 56 bytes */
  };

This now allows to have 13 delayed nodes per 4K page instead of 12.

Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

fs/btrfs/delayed-inode.h

@@ -64,9 +64,9 @@ struct btrfs_delayed_node {
 	struct mutex mutex;
 	struct btrfs_inode_item inode_item;
 	refcount_t refs;
+	int count;
 	u64 index_cnt;
 	unsigned long flags;
-	int count;
 	/*
 	 * The size of the next batch of dir index items to insert (if this
 	 * node is from a directory inode). Protected by @mutex.