Re: [RFC/PATCH 01/15] preparation: provide hook to enable pgstes in user pagetable

Carsten Otte
2008-03-20T20:35:56Z

Dave Hansen wrote:
> Well, and more fundamentally: do we really want dup_mm() able to be
> called from other code?
>
> Maybe we need a bit more detailed justification why fork() itself isn't
> good enough. It looks to me like they basically need an arch-specific
> argument to fork, telling the new process's page tables to take the
> fancy new bit.
>
> I'm really curious how this new stuff is going to get used. Are you
> basically replacing fork() when creating kvm guests?
No. The trick is, that we do need bigger page tables when running
guests: our page tables are usually 2k, but when running a guest
they're 4k to track both guest and host dirty&reference information.
This looks like this:
*----------*
*2k PTE's *
*----------*
*2k PGSTE *
*----------*
We don't want to waste precious memory for all page tables. We'd like
to have one kernel image that runs regular server workload _and_
guests. Therefore, we need to reallocate the page table after fork()
once we know that task is going to be a hypervisor. That's what this
code does: reallocate a bigger page table to accomondate the extra
information. The task needs to be single-threaded when calling for
extended page tables.

Btw: at fork() time, we cannot tell whether or not the user's going to
be a hypervisor. Therefore we cannot do this in fork.