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    SubjectRe: [PATCH 01/11] IMA: use rbtree instead of radix tree for inode information cache
    On Mon, Oct 25, 2010 at 02:41:18PM -0400, Eric Paris wrote:
    > The IMA code needs to store the number of tasks which have an open fd
    > granting permission to write a file even when IMA is not in use. It needs
    > this information in order to be enabled at a later point in time without
    > losing it's integrity garantees. At the moment that means we store a
    > little bit of data about every inode in a cache. We use a radix tree key'd
    > on the inode's memory address. Dave Chinner pointed out that a radix tree
    > is a terrible data structure for such a sparse key space. This patch
    > switches to using an rbtree which should be more efficient.

    I'm not sure this is the right fix, though.

    Realistically, there is a 1:1 relationship between the inode and the
    IMA information. I fail to see why an external index is needed here
    at all - just use a separate structure to store the IMA information
    that the inode points to. That makes the need for a new global index
    and global lock go away completely.

    You're already adding 8 bytes to the inode, so why not make it a
    pointer. We've got 4 conditions:

    1. not configured - no overhead
    2. configured, boot time disabled - 8 bytes per inode
    3. configured, boot time enabled, runtime disabled - 8 bytes per
    inode + small IMA structure
    4. configured, boot time enabled, runtime enabled - 8 bytes per
    inode + large IMA structure

    Anyone who wants the option of runtime enablement can take the extra
    allocation overhead, but otherwise nobody is affected apart from 8
    bytes of additional memory per inode. I doubt that will change
    anything unless it increases the size of the inode enough to push it
    over slab boundaries. And if LSM stacking is introduced, then that 8
    bytes per inode overhead will go away, anyway.

    This approach doesn't introduce new global lock and lookup overhead
    into the main VFS paths, allows you to remove a bunch of code and
    has a path forward for removing the 8 byte per inode overhead as
    well. Seems like the best compromise to me....


    Dave Chinner

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      Last update: 2010-10-26 01:27    [W:0.022 / U:2.276 seconds]
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