Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user_pages()

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Apr 15, 2023 at 12:27:45AM +0100, Lorenzo Stoakes wrote:
> Commit edd478269640 ("io_uring/rsrc: disallow multi-source reg buffers")
> prevents io_pin_pages() from pinning pages spanning multiple VMAs with
> permitted characteristics (anon/huge), requiring that all VMAs share the
> same vm_file.

That commmit doesn't really explain why io_uring is doing such a weird
thing.

What exactly is the problem with mixing struct pages from different
files and why of all the GUP users does only io_uring need to care
about this?

If there is no justification then lets revert that commit instead.

>  		/* don't support file backed memory */
> -		for (i = 0; i < nr_pages; i++) {
> -			if (vmas[i]->vm_file != file) {
> -				ret = -EINVAL;
> -				break;
> -			}
> -			if (!file)
> -				continue;
> -			if (!vma_is_shmem(vmas[i]) && !is_file_hugepages(file)) {
> -				ret = -EOPNOTSUPP;
> -				break;
> -			}
> -		}
> +		file = vma->vm_file;
> +		if (file && !vma_is_shmem(vma) && !is_file_hugepages(file))
> +			ret = -EOPNOTSUPP;
> +

Also, why is it doing this?

All GUP users don't work entirely right for any fops implementation
that assumes write protect is unconditionally possible. eg most
filesystems.

We've been ignoring blocking it because it is an ABI break and it does
sort of work in some cases.

I'd rather see something like FOLL_ALLOW_BROKEN_FILE_MAPPINGS than
io_uring open coding this kind of stuff.

Jason