RE: [RESEND RFC PATCH 0/5] Remote mapping

[Mircea CIRJALIU - MELIU <mcirjaliu@bitdefender.com>]

> On Fri, Sep 04, 2020 at 02:31:11PM +0300, Adalbert Lazăr wrote:
> > VMAs obtained by mmap()ing memory access fds mirror the contents of
> > the remote process address space within the specified range. Pages are
> > installed in the current process page tables at fault time and removed
> > by the mmu_interval_notifier invalidate callbck. No further memory
> management is involved.
> > On attempts to access a hole, or if a mapping was removed by
> > PIDFD_MEM_UNMAP, or if the remote process address space was reaped
> by
> > OOM, the remote mapping fault handler returns VM_FAULT_SIGBUS.
> 
> I still think anything along these lines needs to meet the XPMEM use cases as
> well, we have to have more general solutions for such MM
> stuff:
> 
> https://gitlab.com/hjelmn/xpmem
> 
> However, I think this fundamentally falls into some of the same bad direction
> as xpmem.
> 
> I would much rather see this design copy & clone the VMA's than try to
> mirror the PTEs inside the VMAs from the remote into a single giant VMA and
> somehow split/mirror the VMA ops.

This design was made specifically for virtual machine introspection, where we 
care more about the contents of the address space, rather than the remote VMAs
and their vmops. (Right now only anon pages can be mapped, but I guess
we can extend to pagecache pages as well.) I just used what seemed to be the
common denominator to all page-related operations: range invalidation.
This looks like a general solution.

IMO cloning a VMA in an address space that has a completely different layout
will present its own set of caveats: What happens if the VMA resizes/splits? 
Can you replay all the remote VMA vmops on the clone VMA?

> This is just too weird and fragile to be maintaible over a long term.
> 
> For instance, one of the major bugs in things like xpmem was that they are
> incompatible with get_user_pages(), largely because of this issue.

We support get_user_pages(), that's how we integrate with KVM.
The difference is the page returned will not belong to the current process.

> I feel like I said this already..
> 
> Jason