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    SubjectRe: [PATCH] ext4: remove alignment padding from ext4_inode_info on 64 bit builds
    Richard Kennedy wrote:
    > On Fri, 2010-08-20 at 15:11 -0500, Eric Sandeen wrote:
    >> Richard Kennedy wrote:
    >>> Reorder structure ext4_inode_info to remove 16 bytes of alignment
    >>> padding on 64 bit builds. This shrinks its size from 904 to 888 bytes
    >>> with (CONFIG_EXT4_S_XATTR=y && CONFIG_QUOTA=n).
    >>>
    >>> This will allow this structure to use one fewer cache lines.
    >>>
    >>> Also change type of i_delalloc_reserved_flag to bool to better reflect
    >>> its usage.
    >>>
    >>> compiled & tested on x86_64
    >>
    >> Looks good to me; there is still a bit of padding at the end:
    >>
    >> qsize_t i_reserved_quota; /* 864 8 */
    >> struct list_head i_completed_io_list; /* 872 16 */
    >> spinlock_t i_completed_io_lock; /* 888 4 */
    >>
    >> /* XXX 4 bytes hole, try to pack */
    >>
    >> /* --- cacheline 14 boundary (896 bytes) --- */
    >> ext4_io_end_t * cur_aio_dio; /* 896 8 */
    >> tid_t i_sync_tid; /* 904 4 */
    >> tid_t i_datasync_tid; /* 908 4 */
    >>
    >>
    >> but it wouldn't save a cacheline anyway...
    >>
    >> -Eric
    >>
    > Thanks Eric, yes there is padding but we don't have anything to fill the
    > hole with :(
    >
    > If it becomes important to save space, i_state_flags seem to be unused
    > in ext4 so we could possibly drop that.

    Actually it is used, it's just expertly hidden in:

    EXT4_INODE_BIT_FNS(state, state_flags):

    #define EXT4_INODE_BIT_FNS(name, field) \
    static inline int ext4_test_inode_##name(struct inode *inode, int bit) \
    { \
    return test_bit(bit, &EXT4_I(inode)->i_##field); \
    } \

    etc..

    -Eric



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