Re: [Qemu-devel] [PATCH 05/21] userfaultfd: add vm_userfaultfd_ctx to the vm_area_struct

[Pavel Emelyanov]

> diff --git a/kernel/fork.c b/kernel/fork.c
> index cf65139..cb215c0 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -425,6 +425,7 @@ static int dup_mmap(struct mm_struct *mm, struct 
> mm_struct *oldmm)
>                       goto fail_nomem_anon_vma_fork;
>               tmp->vm_flags &= ~VM_LOCKED;
>               tmp->vm_next = tmp->vm_prev = NULL;
> +             tmp->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;

This creates an interesting effect when the userfaultfd is used outside of
the process which created and activated one. If I try to monitor the memory
usage of one task with another, once the first task fork()-s, its child
begins to see zero-pages in the places where the monitor task was supposed
to insert pages with data.

>               file = tmp->vm_file;
>               if (file) {
>                       struct inode *inode = file_inode(file);
> .
>