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
