Re: RFC: pageable kernel-segments

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <27525795B28BD311B28D00500481B7601F11D9@ftrs1.intranet.ftr.nl>
By author:    "Heusden, Folkert van" <f.v.heusden@ftr.nl>
In newsgroup: linux.dev.kernel
>
> Would anyone be intrested (besides me) in a kernel which can page
> out certain parts of itself? The kernel should be in some kind of
> vmlinux-ish (as in: uncompressed) format on disk for on-demand
> re-loading of pages which are discarded.
> Certain parts of drivers could get the __pageable prefix or so
> (like the __init parts of drivers which get removed) for letting
> the paging-code know that it can be discared if memory-pressure
> demands it.
> __pageable -code would then be things like (e.g.!) the code which
> handles the open()/close() of a device. Most of the time a device
> spends more time doing read/write/ioctl then close/open so. Also;
> hopefully there's no interrupt-sensitive code in these routines.
> I would think is usable (for example) for my 8MB ram laptop.
> Anyone any thoughts on this?
> 

VMS does this.  It at least used to have a great tendency to crash
itself, because it swapped out something that was called from a driver
that was called by the swapper -- resulting in deadlock.  You need
iron discipline for this to work right in all circumstances.

Second, it makes it quite hard to know what operations can cause a
task to sleep, since any reference to paged-out memory can require a
page-in and the associated schedule.  You almost need pointer
annotation in order for this to be safe.

	-hpa
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