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