pin files in memory after read

[Olaf Hering <olh@suse.de>]

Is there a way to always keep a file (once read from disk) in memory, no
matter how much memory pressure exists?
There are always complains that updatedb and similar tools wipe out all
caches. So I guess there is no such thing yet.

I simply want to avoid the spinup of my ibook harddisk when something
has been 'forgotten' and must be loaded again (like opening a new screen
window after a while).

The best I could do so far was a cramfs image. I copied it to tmpfs
during early boot, then mount -o bind every cramfs file over the real
binary on disk. Of course that will fail as soon as I want to update an
affected package because the binary is busy (readonly). So there must be
a better way to achieve this.

How can one tell the kernel to pin a file in memory once it was read?
Maybe with an xattr or something?
Unfortunately I dont know about the block layer and other things
involved, so I cant attach a patch that does what I want.