Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

[Lorenzo Stoakes]

On Mon, May 15, 2023 at 02:03:15PM +0300, Kirill A . Shutemov wrote:
> On Thu, May 04, 2023 at 10:27:50PM +0100, Lorenzo Stoakes wrote:
> > Writing to file-backed mappings which require folio dirty tracking using
> > GUP is a fundamentally broken operation, as kernel write access to GUP
> > mappings do not adhere to the semantics expected by a file system.
> >
> > A GUP caller uses the direct mapping to access the folio, which does not
> > cause write notify to trigger, nor does it enforce that the caller marks
> > the folio dirty.
>
> Okay, problem is clear and the patchset look good to me. But I'm worried
> breaking existing users.
>
> Do we expect the change to be visible to real world users? If yes, are we
> okay to break them?

The general consensus at the moment is that there is no entirely reasonable
usage of this case and you're already running the riks of a kernel oops if
you do this, so it's already broken.

>
> One thing that came to mind is KVM with "qemu -object memory-backend-file,share=on..."
> It is mostly used for pmem emulation.
>
> Do we have plan B?

Yes, we can make it opt-in or opt-out via a FOLL_FLAG. This would be easy
to implement in the event of any issues arising.

>
> Just a random/crazy/broken idea:
>
>  - Allow folio_mkclean() (and folio_clear_dirty_for_io()) to fail,
>    indicating that the page cannot be cleared because it is pinned;
>
>  - Introduce a new vm_operations_struct::mkclean() that would be called by
>    page_vma_mkclean_one() before clearing the range and can fail;
>
>  - On GUP, create an in-kernel fake VMA that represents the file, but with
>    custom vm_ops. The VMA registered in rmap to get notified on
>    folio_mkclean() and fail it because of GUP.
>
>  - folio_clear_dirty_for_io() callers will handle the new failure as
>    indication that the page can be written back but will stay dirty and
>    fs-specific data that is associated with the page writeback cannot be
>    freed.
>
> I'm sure the idea is broken on many levels (I have never looked closely at
> the writeback path). But maybe it is good enough as conversation started?
>

Yeah there are definitely a few ideas down this road that might be
possible, I am not sure how a filesystem can be expected to cope or this to
be reasonably used without dirty/writeback though because you'll just not
track anything or I guess you mean the mapping would be read-only but
somehow stay dirty?

I also had ideas along these lines of e.g. having a special vmalloc mode
which mimics the correct wrprotect settings + does the right thing, but of
course that does nothing to help DMA writing to a GUP-pinned page.

Though if the issue is at the point of the kernel marking the page dirty
unexpectedly, perhaps we can just invoke the mkwrite() _there_ before
marking dirty?

There are probably some sycnhronisation issues there too.

Jason will have some thoughts on this I'm sure. I guess the key question
here is - is it actually feasible for this to work at all? Once we
establish that, the rest are details :)

> --
>   Kiryl Shutsemau / Kirill A. Shutemov