Re: [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder versions

[Andrew Morton]

On Tue, 9 Oct 2018 17:42:09 -0700 John Hubbard <jhubbard@xxxxxxxxxx> wrote:

> > Also, maintainability.  What happens if someone now uses put_page() by
> > mistake?  Kernel fails in some mysterious fashion?  How can we prevent
> > this from occurring as code evolves?  Is there a cheap way of detecting
> > this bug at runtime?
> > 
> 
> It might be possible to do a few run-time checks, such as "does page that came 
> back to put_user_page() have the correct flags?", but it's harder (without 
> having a dedicated page flag) to detect the other direction: "did someone page 
> in a get_user_pages page, to put_page?"
> 
> As Jan said in his reply, converting get_user_pages (and put_user_page) to 
> work with a new data type that wraps struct pages, would solve it, but that's
> an awfully large change. Still...given how much of a mess this can turn into 
> if it's wrong, I wonder if it's worth it--maybe? 

This is a real worry.  If someone uses a mistaken put_page() then how
will that bug manifest at runtime?  Under what set of circumstances
will the kernel trigger the bug?

(btw, please cc me on all patches, not just [0/n]!)