namecache work

Matt Dillon is starting work on cache_lookup(), for at least the next week or so. When implemented, it should lead to some very interesting changes in the filesystem. This may be backportable to FreeBSD, too. Quoted text follows:

“The first step I am taking is to make the namecache more persistent.
ALL active vnodes accessed via lookup() will be guarenteed [sic] to have a
namecache entry. I have successfully booted a test system with this
change, though I am sure there are bugs :-). I am not entirely sure what
to do about the filehandle functions but I am not going to worry about
it for the moment. What this basically means is that vnodes cannot be
recycled in the ‘middle’ of the topology, only at the leafs. This does
not present a big problem since files are always leafs. The namecache
topology will be guarenteed to remain unbroken except in cases where
the namespace is deleted (e.g. removing a file with active descriptors).

The second step will be to use a namecache structural pointer for our
‘directory handle’ in all places in the system where a directory handle
is expected (e.g. ‘dvp’). This will also involve getting rid of the
vnode-based parent directory support (v_dd). Since the namecache
structure has a pointer to the vnode this is a pretty easy step. A
little harder will be to fix all the directory scanning functions to
use the namecache topology instead of the vnode topology.

The third step will be to use the namecache for all name-based locking
operations instead of the underlying vnodes. For example, if you
are renaming a/b to c/d you only need to hold locks on the namecache
entry representing “b” and the one representing “d” prior to executing
the rename operation. The one representing “d” will be a negative
cache entry, placemarking the operation. This will not only completely
solve the locking issues with rename(), remove(), and create, it also
completely solves directory recursion stalls in both directions,
completely solves the race to root issue, solves most of the directory
lookup stalls (cache case lookups will not need a vnode lock and can
run in parallel with directory operations that do need the vnode lock),
gets rid of all name-related deadlock situations, and potentially allows
modifying directory operations to become reentrant down the line.

The fourth step will be a BIG Carrot… the namecache topology does
not have a problem with vnodes appearing in multiple places in the
filesystem. This means that (A) it will be possible to hardlink
directories and (B) it will be possible to implement semi-hard links,
basically softlinks that look like hardlinks and (C) to be able to
CD forwards and backwards without the system getting confused about
paths. In other words, some way cool shit.

Additional optimizations are possible for the future. For example,
it will be possible to cache ucred pointers in the namecache structure
and thus allow namei() to COMPLETELY resolve a path without making
any VOP calls at all, which will at least double and probably quadruple
best case path lookup performance.”

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