From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jann Horn <jannh@google.com>
Subject: Re: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication
 a process mapping
Date: Thu, 16 May 2019 15:32:06 +0200
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To: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Dan Williams <dan.j.williams@intel.com>, Michal Hocko <mhocko@suse.com>, keith.busch@intel.com, "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, pasha.tatashin@oracle.com, Alexander Duyck <alexander.h.duyck@linux.intel.com>, ira.weiny@intel.com, Andrey Konovalov <andreyknvl@google.com>, arunks@codeaurora.org, Vlastimil Babka <vbabka@suse.cz>, Christoph Lameter <cl@linux.com>, Rik van Riel <riel@surriel.com>, Kees Cook <keescook@chromium.org>, hannes@cmpxchg.org, npiggin@gmail.com, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Shakeel Butt <shakeelb@google.com>, Roman Gushchin <guro@fb.com>, Andrea Arcangeli <aarcange@redhat.com>, Hugh Dickins <hughd@google.com>, Jerome Glisse <jglisse@redhat.co>
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On Wed, May 15, 2019 at 5:11 PM Kirill Tkhai <ktkhai@virtuozzo.com> 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?