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

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.