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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Sep 04, 2020 at 01:24:43PM +0000, Mircea CIRJALIU - MELIU wrote:
> > 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.

The point is that a VMA is how the MM connects its parts together,
cloning the content of a VMA without the rest of the VMA meta-data is
just going to be very fragile in the long run.. 

Especially if the VMA is presented as a normal VMA with working struct
pages/etc, not a pfn map.

> 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?

The mirror would have to reclone the source VMA every time the source
VMA changes.

> > 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.

This seems really sketchy, get_user_pages is sensitive to the VMA,
what happens when VMA flags are different/etc?

Jason