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

[Kirill Tkhai]

On 16.05.2019 16:42, Adam Borowski wrote:
> On Thu, May 16, 2019 at 04:10:07PM +0300, Kirill Tkhai wrote:
>> 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.
> 
> fork() manages that, and so does KSM.  Like KSM, you want to make a page
> shared -- you just skip the comparison step as you want to overwrite the old
> contents.
> 
> And there's no need to touch the page, as fork() manages that fine no matter
> if the page is resident, anonymous in swap, or file-backed, all without
> reading from swap.

Yes, and in case of you dive into the patchset, you will found the new syscall
manages page table entries in the same way fork() makes.
 
>>>> 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.
> 
> 
> Meow!
>