Hi, Jann, 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. Kirill
