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

[Kirill A. Shutemov]

On Tue, May 28, 2019 at 12:15:16PM +0300, Kirill Tkhai wrote:
> 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?

I don't see anything immediately wrong with this, but it's still going to
produce puzzling errors for a user. How would you document such limitation
in the way it makes sense for userspace developer?

-- 
 Kirill A. Shutemov