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
