If a filesystem supports POSIX ACL's, the VFS layer expects the filesystem to do POSIX ACL checks on any files not owned by the caller, and it does this for every single pathname component that it looks up.

That obviously can be pretty expensive if the filesystem isn't careful about it, especially with locking. That's doubly sad, since the common case tends to be that there are no ACL's associated with the files in question.

ext3 already caches the ACL data so that it doesn't have to look it up over and over again, but it does so by taking the inode->i_lock spinlock on every lookup. Which is a noticeable overhead even if it's a private lock, especially on CPU's where the serialization is expensive (eg Intel Netburst aka 'P4').

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[ At some point we probably want to move this ACL caching logic entirely
into the VFS layer (and only call down to the filesystem when
uncached), but in the meantime this improves ext3 a bit.

A similar fix to btrfs makes a much bigger difference (15x improvement
in lmbench) due to broken caching. ]