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

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Date:   Wed, 15 May 2019 21:38:41 +0200
From:   Adam Borowski <kilobyte@...band.pl>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     akpm@...ux-foundation.org, dan.j.williams@...el.com,
        mhocko@...e.com, keith.busch@...el.com,
        kirill.shutemov@...ux.intel.com, pasha.tatashin@...cle.com,
        alexander.h.duyck@...ux.intel.com, ira.weiny@...el.com,
        andreyknvl@...gle.com, arunks@...eaurora.org, vbabka@...e.cz,
        cl@...ux.com, riel@...riel.com, keescook@...omium.org,
        hannes@...xchg.org, npiggin@...il.com,
        mathieu.desnoyers@...icios.com, shakeelb@...gle.com, guro@...com,
        aarcange@...hat.com, hughd@...gle.com, jglisse@...hat.com,
        mgorman@...hsingularity.net, daniel.m.jordan@...cle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication
 a process mapping

On Wed, May 15, 2019 at 06:11:15PM +0300, Kirill Tkhai 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:

I wonder, why not optimize the existing interfaces to do zero copy if
properly aligned?  No need for a new syscall, and old code would immediately
benefit.

> There are several problems with process_vm_writev() in this example:
> 
> 1)it causes pagefault on remote process memory, and it forces
>   allocation of a new page (if was not preallocated);
> 
> 2)amount of memory for this example is doubled in a moment --
>   n pages in current and n pages in remote tasks are occupied
>   at the same time;
> 
> 3)received data has no a chance to be properly swapped for
>   a long time.

That'll handle all of your above problems, except for making pages
subject to CoW if written to.  But if making pages writeably shared is
desired, the old functions have a "flags" argument that doesn't yet have a
single bit defined.


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