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    SubjectRe: [patch] epoll use a single inode ...
    On Wed, Mar 07, 2007 at 08:16:14AM +0100, Eric Dumazet wrote:
    > Crazy ideas : (some readers are going to kill me)
    >
    > 1) Use the low order bit of f_path.dentry to say : this pointer is not a
    > pointer to a dentry but the inode pointer (with the low order bit set to 1)
    >
    > OR
    >
    > 2) file->f_path.dentry set to NULL for this special files (so that we dont
    > need to dput() and cache line ping pong the common dentry each time we
    > __fput() a pipe/socket.

    No way on either one. f_path.dentry always beeing there is an assumption
    we make all over the place, and changing that would be a big regression
    for code qualityand reliability all over the place.

    Face it folks, memory is generally cheap, and we're not going to uglify
    huge amounts of code to shave of a little bit.

    [and that is only in reply to this one, the single dentry optimizations
    for epoll and friends are perfecltly fine from the high level view]

    > Same trick could be used for file->f_path.mnt, because there is a big SMP
    > cache line ping/pong to maintain a mnt_count on pipe/sockets mountpoint
    > while these file systems cannot be un-mounted)

    Same thing as above. We might do a hack to not refcount these vfsmounts,
    but we definitively want to keep the invariant of f_path.mnt never
    beeing NULL.

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