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

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

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On Fri, Sep 04, 2020 at 02:31:11PM +0300, Adalbert Lazăr wrote:

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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.

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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

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by

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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.

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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.

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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?

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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.

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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.

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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?

A debugger shouldn't complain if a portion of the debuggee is read-only,
just overwrite the data.

Jason