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

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Apr 17, 2023 at 09:56:34AM -0300, Jason Gunthorpe wrote:
> 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 will leave this to Jens and Pavel to revert on!

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

How would the semantics of this work? What is broken? It is a little
frustrating that we have FOLL_ANON but hugetlb as an outlying case, adding
FOLL_ANON_OR_HUGETLB was another consideration...

> Jason