Re: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication a process mapping

[Jann Horn]

On Thu, May 16, 2019 at 3:03 PM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> On 15.05.2019 21:46, Jann Horn wrote:
> > On Wed, May 15, 2019 at 5:11 PM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> >> This patchset adds a new syscall, which makes possible
> >> to clone a mapping from a process to another process.
> >> The syscall supplements the functionality provided
> >> by process_vm_writev() and process_vm_readv() syscalls,
> >> and it may be useful in many situation.
> >>
> >> For example, it allows to make a zero copy of data,
> >> when process_vm_writev() was previously used:
> > [...]
> >> This syscall may be used for page servers like in example
> >> above, for migration (I assume, even virtual machines may
> >> want something like this), for zero-copy desiring users
> >> of process_vm_writev() and process_vm_readv(), for debug
> >> purposes, etc. It requires the same permittions like
> >> existing proc_vm_xxx() syscalls have.
> >
> > Have you considered using userfaultfd instead? userfaultfd has
> > interfaces (UFFDIO_COPY and UFFDIO_ZERO) for directly shoving pages
> > into the VMAs of other processes. This works without the churn of
> > creating and merging VMAs all the time. userfaultfd is the interface
> > that was written to support virtual machine migration (and it supports
> > live migration, too).
>
> I know about userfaultfd, but it does solve the discussed problem.
> It allocates new pages to make UFFDIO_COPY (see mcopy_atomic_pte()),
> and it accumulates all the disadvantages, the example from [0/5]
> message has.

Sorry, right, I misremembered that.