Messages in this thread | | | Date | Wed, 11 Jun 2008 19:46:20 +0100 | From | Jeremy Fitzhardinge <> | Subject | Re: [rfc][patch] mm: vmap rewrite |
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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
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