Re: [Qemu-devel] [PATCH 19/21] userfaultfd: remap_pages: UFFDIO_REMAP preparation

[Pavel Emelyanov]

> +ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> +                 unsigned long dst_start, unsigned long src_start,
> +                 unsigned long len, __u64 mode)
> +{
> +     struct vm_area_struct *src_vma, *dst_vma;
> +     long err = -EINVAL;
> +     pmd_t *src_pmd, *dst_pmd;
> +     pte_t *src_pte, *dst_pte;
> +     spinlock_t *dst_ptl, *src_ptl;
> +     unsigned long src_addr, dst_addr;
> +     int thp_aligned = -1;
> +     ssize_t moved = 0;
> +
> +     /*
> +      * Sanitize the command parameters:
> +      */
> +     BUG_ON(src_start & ~PAGE_MASK);
> +     BUG_ON(dst_start & ~PAGE_MASK);
> +     BUG_ON(len & ~PAGE_MASK);
> +
> +     /* Does the address range wrap, or is the span zero-sized? */
> +     BUG_ON(src_start + len <= src_start);
> +     BUG_ON(dst_start + len <= dst_start);
> +
> +     /*
> +      * Because these are read sempahores there's no risk of lock
> +      * inversion.
> +      */
> +     down_read(&dst_mm->mmap_sem);
> +     if (dst_mm != src_mm)
> +             down_read(&src_mm->mmap_sem);
> +
> +     /*
> +      * Make sure the vma is not shared, that the src and dst remap
> +      * ranges are both valid and fully within a single existing
> +      * vma.
> +      */
> +     src_vma = find_vma(src_mm, src_start);
> +     if (!src_vma || (src_vma->vm_flags & VM_SHARED))
> +             goto out;
> +     if (src_start < src_vma->vm_start ||
> +         src_start + len > src_vma->vm_end)
> +             goto out;
> +
> +     dst_vma = find_vma(dst_mm, dst_start);
> +     if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
> +             goto out;

I again have a concern about the case when one task monitors the VM of the
other one. If the target task (owning the mm) unmaps a VMA then the monitor
task (holding and operating on the ufd) will get plain EINVAL on UFFDIO_REMAP
request. This is not fatal, but still inconvenient as it will be hard to
find out the reason for failure -- dst VMA is removed and the monitor should
just drop the respective pages with data, or some other error has occurred
and some other actions should be taken.

Thanks,
Pavel