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

[Kirill Tkhai <ktkhai@virtuozzo.com>]

Hi, Adam,

On 15.05.2019 22:38, Adam Borowski wrote:
> 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.

Because, this is just not possible. You can't zero copy anonymous pages
of a process to pages of a remote process, when they are different pages.

>> 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.

Kirill