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

[Christoph Hellwig]

On Wed, May 17, 2023 at 08:55:27AM +0100, Lorenzo Stoakes wrote:
> I'll try to take this in good faith because... yeah. I do get that, I mean
> I literally created a repro for this situation and referenced in the commit
> msg and comments this precise problem in my patch series that
> addresses... this problem :P
> 
> Perhaps I'm not being clear but it was simply my intent to highlight that
> yes this is the primary problem but ALSO GUP writing to ostensibly 'clean'
> pages 'behind the back' of a fs is _also_ a problem.

Yes, it absolutely is a problem if that happens.  But we can just
fix it in the kernel using the:

   lock_page()
   copy data
   set_page_dirty_locked()
   unlock_page();

pattern, and we should have covere every place that did in tree.
But there's no good way to verify it except for regular code audits.