Re: [PATCH 12/18] io_uring: add support for pre-mapped user IO buffers

[Jeff Moyer]

Hi, Jens,

Jens Axboe <ax...@kernel.dk> writes:

> For now, buffers must not be file backed. If file backed buffers are
> passed in, the registration will fail with -1/EOPNOTSUPP. This
> restriction may be relaxed in the future.

[...]

> +             down_write(&current->mm->mmap_sem);
> +             pret = get_user_pages_longterm(ubuf, nr_pages, FOLL_WRITE,
> +                                             pages, vmas);
> +             if (pret == nr_pages) {
> +                     /* don't support file backed memory */
> +                     for (j = 0; j < nr_pages; j++) {
> +                             struct vm_area_struct *vma = vmas[j];
> +
> +                             if (vma->vm_file) {
> +                                     ret = -EOPNOTSUPP;
> +                                     break;
> +                             }
> +                     }

Unfortunately, this suffers the same problem as FOLL_ANON.  Huge pages
are backed by hugetlbfs, and vma->vm_file will be filled in.

I guess you could check is_file_hugepages(vma->vm_file):

        if (vma->vm_file &&
            !is_file_hugepages(vma->vm_file)) {
                ret = -EOPNOTSUPP;
                break;
       }

That works for me.

-Jeff