Re: [9fans] Re: 9P, procedure calls, kernel, confusion. :(

Hi,
[color=blue]
> One more point, I googled a lot on "kernel resident file systems and
> non kernel resident file systems", but I could not find a single
> useful link. It would be great if you could specify the difference
> between the two. I wish that eases up the situation a bit.[/color]

I don't claim to know much about how 9P or Plan 9 work, but I will
attempt to answer.

9P is very much like Linux's VFS. It defines how user-space
applications can access files; whether they are stored locally, or on
the network, or whether the information is generated on-the-fly by the
kernel from its internal data-structures is of no consequence - 9P
abstracts all that.

Accessing files from within the kernel is a different ball-game
(that's true for every kernel). Since you don't have the 9P style
access anymore - The 'Dev' structure essentially replaces it. These
structures point to methods are quite analogous to the 9P operations.
The methods do the work of implementing the file operations - and
these would all be different for files depending on whether they are
stored on local disk, produced synthetically or accessed over a
network. This makes it easier to work with files in the kernel since
all the operations are delegated further to the 'Dev' structures, each
one doing what it knows best; quite similar to what happens with 9P in
user-space.

Charles mentions one such 'Dev', the mount driver, which merely passes
on the received request as 9P messages to the file descriptor
(possibly connected to a remote 9P server).

"Kernel resident filesystem" in this context simply means a filesystem
which was created for use by the kernel; this may or may not be
visible to user-space applications - I'm not too sure. To sum up, you
use the 9 primitive operations provided by each 'Dev' when you work
with kernel-resident filesystems, while all other filesystems are
dealt with using regular 9P.

I hope I'm right, and that this helps.

--
Anant

one thing of note, linux vfs implements a dcache. this connects the
virtual memory system to the filesystem. (but oddly in linux network
buffers are handled seperately.) there is no client-side caching in
the plan 9 kernel.

there is a notable exception. there is an executable cache.
[color=blue][color=green]
>> One more point, I googled a lot on "kernel resident file systems and
>> non kernel resident file systems", but I could not find a single
>> useful link. It would be great if you could specify the difference
>> between the two. I wish that eases up the situation a bit.[/color][/color]

since the devtab[] functions map 1:1 with 9p, all the mount driver
needs to do for calls outside the kernel is to marshal/demarshal
9p messages.

it's important to remember that most in-kernel file servers could easily
exist outside the kernel. the entire ip stack can be implemented from
user space. (and it has been in the past.)
[color=blue]
> "Kernel resident filesystem" in this context simply means a filesystem
> which was created for use by the kernel; this may or may not be
> visible to user-space applications - I'm not too sure.[/color]

every element of mounttab[] has an associated device letter. to
mount the device, one does (typically)
bind -a '#'^$letter /dev

for example, to bind a second ip stack on /net.alt,
bind -a '#I1' /net.alt
[color=blue]
> To sum up, you
> use the 9 primitive operations provided by each 'Dev' when you work
> with kernel-resident filesystems, while all other filesystems are
> dealt with using regular 9P.[/color]

all devices are accessed through devtab. it may be that that entry
is the mount driver. the mount driver turns devtab[]->fn into
the corresponding 9p message. (and vice versa.)

- erik

> there is no client-side caching in the plan 9 kernel.

*cough*, MCACHE?