• Dave Chinner's avatar
    xfs: Replace per-ag array with a radix tree · 1c1c6ebc
    Dave Chinner authored
    The use of an array for the per-ag structures requires reallocation
    of the array when growing the filesystem. This requires locking
    access to the array to avoid use after free situations, and the
    locking is difficult to get right. To avoid needing to reallocate an
    array, change the per-ag structures to an allocated object per ag
    and index them using a tree structure.
    
    The AGs are always densely indexed (hence the use of an array), but
    the number supported is 2^32 and lookups tend to be random and hence
    indexing needs to scale. A simple choice is a radix tree - it works
    well with this sort of index.  This change also removes another
    large contiguous allocation from the mount/growfs path in XFS.
    
    The growing process now needs to change to only initialise the new
    AGs required for the extra space, and as such only needs to
    exclusively lock the tree for inserts. The rest of the code only
    needs to lock the tree while doing lookups, and hence this will
    remove all the deadlocks that currently occur on the m_perag_lock as
    it is now an innermost lock. The lock is also changed to a spinlock
    from a read/write lock as the hold time is now extremely short.
    
    To complete the picture, the per-ag structures will need to be
    reference counted to ensure that we don't free/modify them while
    they are still in use.  This will be done in subsequent patch.
    Signed-off-by: default avatarDave Chinner <david@fromorbit.com>
    Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarAlex Elder <aelder@sgi.com>
    1c1c6ebc
xfs_fsops.c 18.3 KB