On 10/9/18 4:20 PM, Andrew Morton wrote: > On Tue, 9 Oct 2018 10:30:25 +0200 Jan Kara <jack@xxxxxxx> 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? >> >> The same will happen as with any other reference counting bug - the special >> user reference will leak. It will be pretty hard to debug I agree. I was >> thinking about whether we could provide some type safety against such bugs >> such as get_user_pages() not returning struct page pointers but rather some >> other special type but it would result in a big amount of additional churn >> as we'd have to propagate this different type e.g. through the IO path so >> that IO completion routines could properly call put_user_pages(). So I'm >> not sure it's really worth it. > > I'm not really understanding. Patch 3/3 changes just one infiniband > driver to use put_user_page(). But the changelogs here imply (to me) > that every user of get_user_pages() needs to be converted to > s/put_page/put_user_page/. > > Methinks a bit more explanation is needed in these changelogs? > OK, yes, it does sound like the explanation is falling short. I'll work on something clearer. Did the proposed steps in the changelogs, such as: [2] https://lkml.kernel.org/r/20180709080554.21931-1-jhubbard@xxxxxxxxxx Proposed steps for fixing get_user_pages() + DMA problems. help at all, or is it just too many references, and I should write the words directly in the changelog? Anyway, patch 3/3 is a just a working example (which we do want to submit, though), and many more conversions will follow. But they don't have to be done all upfront--they can be done in follow up patchsets. The put_user_page*() routines are, at this point, not going to significantly change behavior. I'm working on an RFC that will show what the long-term fix to get_user_pages and put_user_pages will look like. But meanwhile it's good to get started on converting all of the call sites. thanks, -- John Hubbard NVIDIA
