On Thu, Oct 11, 2018 at 10:49:29AM +0200, Jan Kara wrote: > > 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... At least for the infiniband code being used as an example here we take the struct page from get_user_pages, then stick it in a sgl, and at put_page time we get the page back out of the sgl via sg_page() So type safety will not help this case... I wonder how many other users are similar? I think this is a pretty reasonable flow for DMA with user pages. Jason
