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

[Jann Horn]

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.
[...]
> The proposed syscall aims to introduce an interface, which
> supplements currently existing process_vm_writev() and
> process_vm_readv(), and allows to solve the problem with
> anonymous memory transfer. The above example may be rewritten as:
>
>         void *buf;
>
>         buf = mmap(NULL, n * PAGE_SIZE, PROT_READ|PROT_WRITE,
>                    MAP_PRIVATE|MAP_ANONYMOUS, ...);
>         recv(sock, buf, n * PAGE_SIZE, 0);
>
>         /* Sign of @pid is direction: "from @pid task to current" or vice versa. */
>         process_vm_mmap(-pid, buf, n * PAGE_SIZE, remote_addr, PVMMAP_FIXED);
>         munmap(buf, n * PAGE_SIZE);

In this specific example, an alternative would be to splice() from the
socket into /proc/$pid/mem, or something like that, right?
proc_mem_operations has no ->splice_read() at the moment, and it'd
need that to be more efficient, but that could be built without
creating new UAPI, right?

But I guess maybe your workload is not that simple? What do you
actually do with the received data between receiving it and shoving it
over into the other process?