Re: [rfc][patch] mm: vmap rewrite

Jeremy Fitzhardinge
2008-06-11T18:47:35Z

Nick Piggin wrote:
> It's harder than that even, because we don't own the page flags, so then
> clearing the PG_kalias bit would require that we make all page flags ops
> atomic in all parts of the kernel. Obviously not going to happen.
>
> The other thing we could do is have vmap layer keep some p->v translations
> around (actually it doesn't even need to go all the way to v, just a single
> bit would suffice) So I guess this would be like another page flag, but
> without the atomicity problem and without me getting angry at using another
> flag ;) Still, I'd rather not do this and slow everything else down.
>

Yeah. It's a bit awkward to maintain a secondary structure just to deal
with the confluence of two edge cases (running Xen + reusing an aliased
page in a pagetable).

> It could be switched on at runtime if Xen is running perhaps. Or the other
> thing Xen could do is keep a cache of unaliased page table pages. You
> could fill it up N pages at a time, and just do a single unmap_aliases call
> to sanitize them all; also, clean pages returned from pagetables could be
> reused. Like the quicklists things.
>

Hm, that wouldn't be too bad (so long as it doesn't end up hiding
gigabytes of memory away from the rest of the system ;).

> Or: doesn't the host have to do its own alias check anyway? In case of an
> AWOL guest? Why not just reuse that and trap back into the guest to fix it
> up?

That's possible, but awkward. In many cases these updates will be
batched, so it would become a matter of issuing a batch, then picking
through the results to see what worked and what failed. I suppose I
could just do the simple flush and then if that turns out too expensive,
do the submit-and-retry approach.

J