Re: Filepaths in VM map for tmpfs files

On Tuesday, February 03, 2015 10:33:36 PM Konstantin Belousov wrote:
> On Mon, Feb 02, 2015 at 09:50:22PM -0600, Eric Badger wrote:
> > On 02/02/2015 03:30 AM, Konstantin Belousov wrote:
> > > On Sun, Feb 01, 2015 at 08:38:29PM -0600, Eric Badger wrote:
> > >> On 01/31/2015 09:36 AM, Konstantin Belousov wrote:
> > >>> First, shouldn't the kve_type changed to KVME_TYPE_VNODE as well ?
> > >> 
> > >> My thinking is no, because KVME_TYPE_SWAP is in fact the correct type;
> > >> I'd opine that it is better to be transparent than make it look like
> > >> there is an OBJT_VNODE object there. It may be that some programs would
> > >> be confused by VNODE info returned on a SWAP type mapping, though I
> > >> know
> > >> that dtrace handles it OK.
> > > 
> > > kve_vn_* and kve_path fields are defined only for KVME_TYPE_VNODE
> > > kve_type.
> > > So this is in fact a bug in whatever used the API to access kve_path
> > > for KVE_TYPE_SWAP.
> > 
> > Hmm, is that documented anywhere? I think it's fair to assume that
> > kve_vn* applies only to the VNODE type,
> > but I know there are several in-tree users that reference kve_path
> > regardless of type (ostensibly relying
> > on the default of an empty string). Maybe one could determine the
> > validity of the kve_vn* fields by
> > inspecting the kve_vn_type (not sure of all the consequences of that)?
> > Or change it to KVME_TYPE_VNODE
> > and deal with the below problem...
> 
> There is no useful documentation for the kern.proc. sysctls.
> My word (and statements from other involved developers) could be
> considered as close to the truth as it can be.
> Somebody taking the efforts to document the stuff would make very
> valuable contribution.

I think that kve_path should be valid for all types (e.g. shm_open() is not a 
vnode but has a pathname, and that should be fixed to display if possible).  
In the equivalent for files (kinfo_file), the pathname is type-independent and 
always valid.

That said, I think tmpfs nodes should be exposed as files.  It is an 
implementation detail of tmpfs that they are swap-backed, but from a user's 
perspective these are files, and if you want to expose other vnode-specific 
fields than just the path, KVME_TYPE_VNODE would be more correct.

-- 
John Baldwin