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

[Kirill A. Shutemov]

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.

One possible way out would be to force CoW on all pages in the mapping,
before passing the mapping to the new process.

Thanks,
Kirill.