On 28.05.2019 19:15, Kirill A. Shutemov wrote: > 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? It's difficult, since the limitation is artificial. I just may to suggest more strict limitation. Something like "VMA may be remapped only as a whole region, and only in the case of there were not fork() after VMA appeared in a process (by mmap or remapping from another remote process). In case of VMA were merged with a neighbouring VMA, the same rules are applied to the neighbours. diff --git a/include/linux/mm.h b/include/linux/mm.h index 0e8834ac32b7..0bcd6f598e73 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -287,13 +287,17 @@ extern unsigned int kobjsize(const void *objp); #define VM_HIGH_ARCH_BIT_2 34 /* bit only usable on 64-bit architectures */ #define VM_HIGH_ARCH_BIT_3 35 /* bit only usable on 64-bit architectures */ #define VM_HIGH_ARCH_BIT_4 36 /* bit only usable on 64-bit architectures */ +#define VM_HIGH_ARCH_BIT_5 37 /* bit only usable on 64-bit architectures */ #define VM_HIGH_ARCH_0 BIT(VM_HIGH_ARCH_BIT_0) #define VM_HIGH_ARCH_1 BIT(VM_HIGH_ARCH_BIT_1) #define VM_HIGH_ARCH_2 BIT(VM_HIGH_ARCH_BIT_2) #define VM_HIGH_ARCH_3 BIT(VM_HIGH_ARCH_BIT_3) #define VM_HIGH_ARCH_4 BIT(VM_HIGH_ARCH_BIT_4) +#define VM_HIGH_ARCH_5 BIT(VM_HIGH_ARCH_BIT_5) #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */ +#define VM_MAY_REMOTE_REMAP VM_HIGH_ARCH_5 + #ifdef CONFIG_ARCH_HAS_PKEYS # define VM_PKEY_SHIFT VM_HIGH_ARCH_BIT_0 # define VM_PKEY_BIT0 VM_HIGH_ARCH_0 /* A protection key is a 4-bit value */ diff --git a/kernel/fork.c b/kernel/fork.c index ff4efd16fd82..a3c758c8cd54 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -584,8 +584,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, rb_parent = &tmp->vm_rb; mm->map_count++; - if (!(tmp->vm_flags & VM_WIPEONFORK)) + if (!(tmp->vm_flags & VM_WIPEONFORK)) { retval = copy_page_range(mm, oldmm, mpnt); + mpnt->vm_flags &= ~VM_MAY_REMOTE_REMAP; + } if (tmp->vm_ops && tmp->vm_ops->open) tmp->vm_ops->open(tmp);
