On 24.05.2019 14:52, Kirill A. Shutemov wrote:

On Fri, May 24, 2019 at 01:45:50PM +0300, Kirill Tkhai wrote:

quoted

On 22.05.2019 18:22, Kirill A. Shutemov wrote:

quoted

On Mon, May 20, 2019 at 05:00:01PM +0300, Kirill Tkhai wrote:

quoted

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).

Thanks,
Kirill