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

[Pavel Begunkov]

On 4/18/23 17:25, Pavel Begunkov wrote:
> On 4/17/23 13:56, 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?
>
> Simply because it doesn't seem sane to mix and register buffers of
> different "nature" as one. It's not a huge deal for currently allowed
> types, e.g. mixing normal and huge anon pages, but it's rather a matter
> of time before it gets extended, and then I'll certainly become a
> problem. We've been asked just recently to allow registering bufs
> provided mapped by some specific driver, or there might be DMA mapped
> memory in the future.
>
> Rejecting based on vmas might be too conservative, I agree and am all
> for if someone can help to make it right.

For some reason I thought it was rejecting if involves more than
one different vma. ->vm_file checks still sound fair to me, but in
any case, open to changing it.

--
Pavel Begunkov