This little bugger exists because of tmpfs.

I have always wanted to create a virtual memory filesystem, like
Solaris' tmpfs.  Currently, doing this involves sticking your hands DEEP
into the guts of the page cache, swap cache, and other nastiness.  The
main reason such nastiness is necessary, AFAICS, is that swappable pages
are tied a process fairly closely.  ie. current->mm is referenced quite
a bit in the MM/swap code.

For tmpfs and other uses the kernel may have for VM, we want driver<->VM
association, not process<->VM association.  So, I hacked up some code
which allows kernel drivers to use swappable VM too.

The main idea behind the hack is to create kernel threads, and associate
MAP_ANONYMOUS mappings with those kernel threads, at the request of any
random kernel driver.

I think this hack makes it possible to use the page cache and swap cache
as a general cache, without having to rewrite the MM subsystem or adding
memory-pressure callbacks or stuff like that.  It seems like filesystems
would find kvmd most useful, but there are probably others uses for it
as well.

Attached is the kernel api (kvmd.h) and sample implementation for this
idea (kvmd.c).  The API can be summarized as

kvmd_open - attach to a VM kernel thread
kvmd_alloc - allocate some swappable memory (create new MAP_ANONYMOUS
mapping)
kvmd_map - fault a region of swappable memory into RAM, for subsequent
access by kernel driver

kvmd_unmap - release region locked into RAM by kvmd_map
kvmd_free - release MAP_ANONYMOUS mapping
kvmd_close - release attachment to VM kernel thread

Random notes...
* A thread pool is used to distribute the mappings, and in the process
create a nicely parallel system
* It would probably be nice to be able to expand and shrink mappings
too.
* It would be simple to expand the system to support apache-style
min/max process thresholds

Disclaimers...
* The big disclaimer :) I am not am MM guru.  This may not solve the
problems described at all...
* THE ATTACHED CODE EXISTS ONLY TO ILLUSTRATE A POINT.  It was typed
straight off-the-cuff, and I never even attempted to compile it.
* the SMP locking may be off, and in some places it might be good to
replace wait queue with semaphore

Comments welcome...

-- 
Jeff Garzik              | Tact is the ability to tell a man 
Building 1024            | he has an open mind when he has a
MandrakeSoft, Inc.       | hole in his head.  (-random fortune)