Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

Kyle Moffett
2005-11-06T08:45:25Z

On Nov 4, 2005, at 10:31:48, Linus Torvalds wrote:
> I can pretty much guarantee that any kernel I maintain will never
> have dynamic kernel pointers: when some memory has been allocated
> with kmalloc() (or equivalent routines - pretty much _any_ kernel
> allocation), it stays put.

Hmm, this brings up something that I haven't seen discussed on this
list (maybe a long time ago, but perhaps it should be brought up
again?). What are the pros/cons to having a non-physically-linear
kernel virtual memory space? Would it be theoretically possible to
allow some kind of dynamic kernel page swapping, such that the _same_
kernel-virtual pointer goes to a different physical memory page?
That would definitely satisfy the memory hotplug people, but I don't
know what the tradeoffs would be for normal boxen.

It seems like the trick would be to make sure that page accesses
_during_ the swap are correctly handled. If the page-swapper
included code in the kernel fault handler to notice that a page was
in the process of being swapped out/in by another CPU, it could just
wait for swap-in to finish and then resume from the new page. This
would get messy with DMA and non-cpu memory accessors and such, which
are what I assume the reasons for not implementing this in the past
have been.

From what I can see, the really dumb-obvious-slow method would be to
call the first and last parts of software-suspend. As memory hotplug
is a relatively rare event, this would probably work well enough
given the requirements:
1) Run software suspend pre-memory-dump code
2) Move pages off the to-be-removed node, remapping the kernel
space to the new locations.
3) Mark the node so that new pages don't end up on it
4) Run software suspend post-memory-reload code

<random-guessing>
Perhaps the non-contiguous memory support would be of some help here?
</random-guessing>

Cheers,
Kyle Moffett

--
Simple things should be simple and complex things should be possible
-- Alan Kay