RFC: New VOP to translate vnode to its component name

[Alexander Kabaev]

On Sun, 07 Dec 2008 11:26:08 -0500
Joe Marcus Clarke <marcus at FreeBSD.org> wrote:

> Background:
> 
> Procstat (i.e. kinfo_file) was a great addition which allows userland
> processes to get a list of open files for a process without the need
> for elevated privileges (e.g. kmem access).  This feature uses the
> VFS cache to find component names from vnodes in a process' file
> descriptor table. Because of its ease of use, I quickly deployed it
> into libgtop so that it could provide an lsof-like feature for
> FreeBSD.
> 
> Another need arose that seemed perfect for procstat: the ability to
> find out what process had the various mouse devices open.  This was
> needed for X.Org's HAL integration.  Unfortunately, due to the fact
> that devfs did not make use of the VFS cache, this was impossible to
> do without bringing it a lot of kvm code from fstat, or simply
> exec'ing fstat periodically.  I chose the latter.  The consequence is
> easier-to-read code, but a performance hit with default HAL
> configurations.
> 
> Robert Watson suggested I teach the VFS cache lookup function to query
> file systems directly when cache lookups fail.  After a few false
> starts, and with the help of kib, I think I have a committable
> implementation.
> 
> Solution:
> 
> Here is a patch to HEAD, along with a man page, for VOP_CNP.  VOP_CNP
> translates a vnode to its component name.  It is currently called from
> vn_fullpath1() to traverse a vnode hierarchy when cache lookups for
> those vnodes fail.  I have currently implemented VOP_CNP for devfs and
> pseudofs.  Kostik has thoroughly reviewed the devfs implementation.  I
> only recently did the pseudofs implementation at his request.
> Additionally, the devfs implementation has gone through a Peter Holm
> stress test, and survives (the pseudofs implementation survives
> WITNESS and VFS lock debugging).
> 
> I would like to commit this work with a possible MFC to RELENG_7 to
> come later.
> 
> http://www.marcuscom.com/downloads/vop_cnp_10.diff
> http://www.marcuscom.com/downloads/VOP_CNP.9
> 
> Joe
> 
In general, the relationship between vnode and componentnames is not
1:1, so I do not see how this VOP can possibly be made a permanent part
of our VFS interface, as its definition is bogus by design.

-- 
Alexander Kabaev
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