Re: ZFS, procfs and java

On Fri, 20 Apr 2007, Stephane E. Potvin wrote:

> It seems that the ZFS cache (dnlc) doesn't update the VSF_namecache fields 
> in the vnodes (v_cache_src, v_cache_dst and v_dd). This unfortunately causes 
> the file link in /proc/curproc to return "unknown" as it uses vn_fullpath 
> which relies on the namecache fields to be set correctly.
>
> Java uses /proc/curproc/file to know where to find its libraries and doesn't 
> expect the "unknown" return value returned when using ZFS (due to 
> vn_fullpath failing). The attached patches to the java/jdk15 port fixes this 
> issue but it doesn't fix the root of the problem.
>
> Would it be possible to add support in dnlc for updating the VFS namecache 
> fields or to add knowledge of dnlc into vn_fullpath?

No application should rely on the correctness and usefulness of 
/proc/curproc/file -- the conversion from a vnode to a pathname is inherently 
unreliable.  Likewise, properly ported applications should not rely on the 
availability of procfs, as its use is deprecated.  My advice would be to talk 
to the Java porters about entirely eliminating this dependency.

As a matter of debugging/logging convenience, it would be nice if ZFS 
implemented the name cache hooks, since using vn_fullpath is useful for procfs 
monitoring of process address space, audit, etc.

Robert N M Watson
Computer Laboratory
University of Cambridge