Re: [Qemu-devel] [PATCH 10/17] mm: rmap preparation for remap_anon_pages

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Linus Torvalds (torvalds@linux-foundation.org) wrote:
> On Mon, Oct 6, 2014 at 12:41 PM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> >
> > Of course if somebody has better ideas on how to resolve an anonymous
> > userfault they're welcome.
> 
> So I'd *much* rather have a "write()" style interface (ie _copying_
> bytes from user space into a newly allocated page that gets mapped)
> than a "remap page" style interface

Something like that might work for the postcopy case; it doesn't work
for some of the other uses that need to stop a page being changed by the
guest, but then need to somehow get a copy of that page internally to QEMU,
and perhaps provide it back later.  remap_anon_pages worked for those cases
as well; I can't think of another current way of doing it in userspace.

I'm thinking here of systems for making VMs with memory larger than a single
host; that's something that's not as well thought out.  I've also seen people
writing emulation that want to trap and emulate some page accesses while
still having the original data available to the emulator itself.

So yes, OK for now, but the result is less general.

Dave


> remapping anonymous pages involves page table games that really aren't
> necessarily a good idea, and tlb invalidates for the old page etc.
> Just don't do it.
> 
>            Linus
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK