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

[Jan Kara]

On Wed 10-10-18 16:45:41, Andrew Morton wrote:
> 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?

At runtime such bug will manifest as a page that can never be evicted from
memory. We could warn in put_page() if page reference count drops below
bare minimum for given user pin count which would be able to catch some
issues but it won't be 100% reliable. So at this point I'm more leaning
towards making get_user_pages() return a different type than just
struct page * to make it much harder for refcount to go wrong...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR