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
