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SubjectRe: [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder versions
On Tue, 9 Oct 2018 17:42:09 -0700 John Hubbard <jhubbard@nvidia.com> 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]!)

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