Re: [PATCH v2 02/10] btrfs: reduce size of struct btrfs_block_group

[Sun YangKai <sunk67188@gmail.com>]

>    struct btrfs_block_group {
>          struct btrfs_fs_info *     fs_info;              /*     0     8 */
>          struct btrfs_inode *       inode;                /*     8     8 */
>          spinlock_t                 lock __attribute__((__aligned__(4))); /*    16     4 */
>          unsigned int               ro;                   /*    20     4 */
>          u64                        start;                /*    24     8 */
>          u64                        length;               /*    32     8 */
>          u64                        pinned;               /*    40     8 */
>          u64                        reserved;             /*    48     8 */
>          u64                        used;                 /*    56     8 */
>          /* --- cacheline 1 boundary (64 bytes) --- */
>          u64                        delalloc_bytes;       /*    64     8 */
>          u64                        bytes_super;          /*    72     8 */
>          u64                        flags;                /*    80     8 */
>          u64                        cache_generation;     /*    88     8 */
>          u64                        global_root_id;       /*    96     8 */
>          u64                        remap_bytes;          /*   104     8 */
>          u32                        identity_remap_count; /*   112     4 */
>          u32                        last_identity_remap_count; /*   116     4 */
>          u64                        last_used;            /*   120     8 */
>          /* --- cacheline 2 boundary (128 bytes) --- */
>          u64                        last_remap_bytes;     /*   128     8 */
>          u64                        last_flags;           /*   136     8 */
>          u32                        bitmap_high_thresh;   /*   144     4 */
>          u32                        bitmap_low_thresh;    /*   148     4 */
>          struct rw_semaphore        data_rwsem __attribute__((__aligned__(8))); /*   152    40 */
>          /* --- cacheline 3 boundary (192 bytes) --- */
>          long unsigned int          full_stripe_len;      /*   192     8 */
>          long unsigned int          runtime_flags;        /*   200     8 */
>          int                        disk_cache_state;     /*   208     4 */
This seems always used as enum btrfs_disk_cache_state
>          int                        cached;               /*   212     4 */
This seems always used as enum btrfs_caching_type

Maybe we could change them to use the more proper type in this patch 
series for better type safety and self documenting.

Thanks,
Sun YangKai
>          struct btrfs_caching_control * caching_ctl;      /*   216     8 */
>          struct btrfs_space_info *  space_info;           /*   224     8 */
>          struct btrfs_free_space_ctl * free_space_ctl;    /*   232     8 */
>          struct rb_node             cache_node __attribute__((__aligned__(8))); /*   240    24 */
>          /* --- cacheline 4 boundary (256 bytes) was 8 bytes ago --- */
>          struct list_head           list;                 /*   264    16 */
>          refcount_t                 refs __attribute__((__aligned__(4))); /*   280     4 */
>          atomic_t                   frozen __attribute__((__aligned__(4))); /*   284     4 */
>          struct list_head           cluster_list;         /*   288    16 */
>          struct list_head           bg_list;              /*   304    16 */
>          /* --- cacheline 5 boundary (320 bytes) --- */
>          struct list_head           ro_list;              /*   320    16 */
>          struct list_head           discard_list;         /*   336    16 */
>          int                        discard_index;        /*   352     4 */
>          enum btrfs_discard_state   discard_state;        /*   356     4 */
>          u64                        discard_eligible_time; /*   360     8 */
>          u64                        discard_cursor;       /*   368     8 */
>          struct list_head           dirty_list;           /*   376    16 */
>          /* --- cacheline 6 boundary (384 bytes) was 8 bytes ago --- */
>          struct list_head           io_list;              /*   392    16 */
>          struct btrfs_io_ctl        io_ctl;               /*   408    72 */
>          /* --- cacheline 7 boundary (448 bytes) was 32 bytes ago --- */
>          atomic_t                   reservations __attribute__((__aligned__(4))); /*   480     4 */
>          atomic_t                   nocow_writers __attribute__((__aligned__(4))); /*   484     4 */
>          struct mutex               free_space_lock __attribute__((__aligned__(8))); /*   488    32 */
>          /* --- cacheline 8 boundary (512 bytes) was 8 bytes ago --- */
>          bool                       using_free_space_bitmaps; /*   520     1 */
>          bool                       using_free_space_bitmaps_cached; /*   521     1 */
> 
>          /* XXX 2 bytes hole, try to pack */
>          /* Bitfield combined with previous fields */
> 
>          static enum btrfs_block_group_size_class size_class; /*     0: 0  0 */
>          int                        swap_extents;         /*   524     4 */
>          u64                        alloc_offset;         /*   528     8 */
>          u64                        zone_unusable;        /*   536     8 */
>          u64                        zone_capacity;        /*   544     8 */
>          u64                        meta_write_pointer;   /*   552     8 */
>          struct btrfs_chunk_map *   physical_map;         /*   560     8 */
>          struct list_head           active_bg_list;       /*   568    16 */
>          /* --- cacheline 9 boundary (576 bytes) was 8 bytes ago --- */
>          struct work_struct         zone_finish_work;     /*   584    32 */
>          struct extent_buffer *     last_eb;              /*   616     8 */
>          u64                        reclaim_mark;         /*   624     8 */
> 
>          /* size: 632, cachelines: 10, members: 60, static members: 1 */
>          /* sum members: 630, holes: 1, sum holes: 2 */
>          /* sum bitfield members: 8 bits (1 bytes) */
>          /* forced alignments: 8 */
>          /* last cacheline: 56 bytes */
> 
>          /* BRAIN FART ALERT! 632 bytes != 630 (member bytes) + 8 (member bits) + 2 (byte holes) + 0 (bit holes), diff = -8 bits */
>    } __attribute__((__aligned__(8)));
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>   fs/btrfs/block-group.h | 33 ++++++++++++++++-----------------
>   1 file changed, 16 insertions(+), 17 deletions(-)