Re: [PATCH v2 0/7] mm: process_vm_mmap() -- syscall for duplication a process mapping

[Kirill Tkhai]

On 28.05.2019 02:30, Kirill A. Shutemov wrote:
> On Fri, May 24, 2019 at 05:00:32PM +0300, Kirill Tkhai wrote:
>> On 24.05.2019 14:52, Kirill A. Shutemov wrote:
>>> On Fri, May 24, 2019 at 01:45:50PM +0300, Kirill Tkhai wrote:
>>>> On 22.05.2019 18:22, Kirill A. Shutemov wrote:
>>>>> On Mon, May 20, 2019 at 05:00:01PM +0300, Kirill Tkhai wrote:
>>>>>> This patchset adds a new syscall, which makes possible
>>>>>> to clone a VMA from a process to current process.
>>>>>> The syscall supplements the functionality provided
>>>>>> by process_vm_writev() and process_vm_readv() syscalls,
>>>>>> and it may be useful in many situation.
>>>>>
>>>>> Kirill, could you explain how the change affects rmap and how it is safe.
>>>>>
>>>>> My concern is that the patchset allows to map the same page multiple times
>>>>> within one process or even map page allocated by child to the parrent.
>>>>>
>>>>> It was not allowed before.
>>>>>
>>>>> In the best case it makes reasoning about rmap substantially more difficult.
>>>>>
>>>>> But I'm worry it will introduce hard-to-debug bugs, like described in
>>>>> https://lwn.net/Articles/383162/.
>>>>
>>>> Andy suggested to unmap PTEs from source page table, and this make the single
>>>> page never be mapped in the same process twice. This is OK for my use case,
>>>> and here we will just do a small step "allow to inherit VMA by a child process",
>>>> which we didn't have before this. If someone still needs to continue the work
>>>> to allow the same page be mapped twice in a single process in the future, this
>>>> person will have a supported basis we do in this small step. I believe, someone
>>>> like debugger may want to have this to make a fast snapshot of a process private
>>>> memory (when the task is stopped for a small time to get its memory). But for
>>>> me remapping is enough at the moment.
>>>>
>>>> What do you think about this?
>>>
>>> I don't think that unmapping alone will do. Consider the following
>>> scenario:
>>>
>>> 1. Task A creates and populates the mapping.
>>> 2. Task A forks. We have now Task B mapping the same pages, but
>>> write-protected.
>>> 3. Task B calls process_vm_mmap() and passes the mapping to the parent.
>>>
>>> After this Task A will have the same anon pages mapped twice.
>>
>> Ah, sure.
>>
>>> One possible way out would be to force CoW on all pages in the mapping,
>>> before passing the mapping to the new process.
>>
>> This will pop all swapped pages up, which is the thing the patchset aims
>> to prevent.
>>
>> Hm, what about allow remapping only VMA, which anon_vma::rb_root contain
>> only chain and which vma->anon_vma_chain contains single entry? This is
>> a vma, which were faulted, but its mm never were duplicated (or which
>> forks already died).
> 
> The requirement for the VMA to be faulted (have any pages mapped) looks
> excessive to me, but the general idea may work.
> 
> One issue I see is that userspace may not have full control to create such
> VMA. vma_merge() can merge the VMA to the next one without any consent
> from userspace and you'll get anon_vma inherited from the VMA you've
> justed merged with.
> 
> I don't have any valid idea on how to get around this.

Technically it is possible by creating boundary 1-page VMAs with another protection:
one above and one below the desired region, then map the desired mapping. But this
is not comfortable.

I don't think it's difficult to find a natural limitation, which prevents mapping
a single page twice if we want to avoid this at least on start. Another suggestion:

prohibit to map a remote process's VMA only in case of its vm_area_struct::anon_vma::root
is the same as root of one of local process's VMA.

What about this?

Thanks,
Kirill