Re: v9fs writepage

[Eric Van Hensbergen <ericvh@gmail.com>]

On 4/30/05, Al Viro <viro@parcelfarce.linux.theplanet.co.uk> wrote:
> On Sat, Apr 30, 2005 at 01:10:39PM -0500, Eric Van Hensbergen wrote:
> 
> > It would seem a relaxed check in my v9fs_fid_locate  would provide the
> > same service as keeping open files in the inode.  We had looked at
> > doing something similar, but I hated the idea of having to keep the
> > extra book-keeping if there was another path (particularly since we
> > are already cacheing fids in dentries).
> 
> a) 9P server is entirely within its rights to serve different contents
> depending on the implied uid and even on phase of moon during the processing
> of auth.  So client-side file contents cache is not kosher.
>

Yes. I agree.  I actually explicitly took all the cacheing behavior
out of v9fs when I started getting involved a year ago.  I only put
the address_space operations back in recently in order to support mmap
(which isn't supported at all under Plan 9).  Would you rather I
remove the vfs_addr.c support in order to stay more Plan 9 consistent?
 I figured I'd have a better chance of getting LKML acceptance by
providing as many "linux semantics" as I could -- specifically we
really wanted to pass fsx.

> 
> Putting the information about caching into 9P would be interesting, but
> that doesn't belong here - such stuff should be taken to Plan 9 maillist.
> 

There is some precident for this in the cfs(4) file system under Plan
9 which providing something similar to the new Linux CacheFS for
non-synthetic file systems (a unofficial rule of thumb was that
anything that had a zero modification time was most likely synthetic
and therefore unsuitable to cache).  My plan has been to start by
integrating this sort of layer on-top of the existing v9fs base and
then do what I can to remove any performance penalty from implementing
it as a stackable layer.  I was also planning on talking to the linux
CacheFS guys to see if their stuff would be suitable for this
application.  It's on the roadmap, but was planning to start after we
get the inital core accepted to the kernel (trying to keep things as
simple as possible).  Its very likely we could just provide all mmap
support within the cache layer.

      -eric