Put a timeout on -ve name cache entries in NFS

[Bruce Evans]

On Mon, 19 Oct 2009, John Baldwin wrote:

> This patch allows one to put an upper time limit on how long the NFS client
> should keep negative cache entries around for a given directory.  This is
> basically a safety belt as there are certain races with -ve entries that are
> not easily fixed (e.g. dealing with the low resolution of the directory
> modification timestamps).  However, timing out the entries would put a limit
> on how stale the client's cache entries could be.  My main question is do
> folks think this value should be tunable as a global sysctl or a per-mount
> option.  A sysctl is far easier to implement (and the acccess cache timeout
> is a sysctl).  However, the other namecache-related settings are all mount
> options, so a mount option might be more consistent.  Current rough patch is
> below:

One reason that I never committed my port of NetBSD's implementation of
negative cache entries for nfs is that I thought the timeout should be
a mount option but I didn't want to deal with the portability problems
from that.

Bruce