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

From: Kirill Tkhai
Date: Thu May 16 2019 - 09:05:02 EST

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.