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Subject: Re: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication
 a process mapping
To:     Jann Horn <jannh@google.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>,
        shakeelb@google.com, Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        daniel.m.jordan@oracle.com,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
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 <CAG48ez20Nu76Q8Tye9Hd3HGCmvfUYH+Ubp2EWbnhLp+J6wqRvw@mail.gmail.com>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <456c7367-0656-933b-986d-febdcc5ab98e@virtuozzo.com>
Date:   Thu, 16 May 2019 16:02:55 +0300
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Hi, Jann,

On 15.05.2019 21:46, Jann Horn wrote:
> 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.
>>
>> 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