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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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.

-- 
 Kirill A. Shutemov