Per-open file private data for the cdevs

[John Baldwin]

On Thursday 15 May 2008 08:30:55 am Kostik Belousov wrote:
> On Thu, May 15, 2008 at 07:30:07AM -0400, John Baldwin wrote:
> > On Wednesday 14 May 2008 05:32:23 am Kostik Belousov wrote:
> > > So, I cannot implement overlay of the cdevpriv over the seqaccess data
> > > without some additional flag. On the other hand, use of 2 bytes for the
> > > f_type is overkill when the range of the types is [1,8]. I divided it
> > > to two one-byte fields, and new field is used for filetype-specific
> > > flags.
> > >
> > > http://people.freebsd.org/~kib/misc/fdpriv.4.patch
> > > shall give the same size of the struct file while not causing
> > > unconditional acquire of the cdevpriv_mtx on the last file close.
> > >
> > > Jeff, do you agree with the proposed vivisection of the f_type ? I will
> > > ask Peter to retest the patch then.
> >
> > I would rather add the new pointer to struct file and avoid breaking the
> > ABI of fstat.  That is, I would like this patch to be MFCable, but you
> > can't MFC this w/o breaking the ABI of struct file since fstat(1) will be
> > reading the full short to get the DTYPE.
>
> Oh, I thought that fstat uses struct xfile, at least for live systems.
>
> I considered to introduce new f_type value for devfs files; the
> DTYPE_VNODE is not a complete truth due to custom f_ops. But DTYPE_VNODE
> is special-cased in enough locations to make this much less preferrable
> then another pointer.

fstat always uses libkvm and direct access unfortunately.  Also, I greatly 
care about fstat on crash dumps as well. :)

-- 
John Baldwin