On Thu, Oct 11, 2018 at 06:23:24PM -0700, John Hubbard wrote: > On 10/11/18 6:20 AM, Jason Gunthorpe wrote: > > 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. > > > > That is true. The infiniband code, fortunately, never mixes the two page > types into the same pool (or sg list), so it's actually an easier example > than some other subsystems. But, yes, type safety doesn't help there. I can > take a moment to look around at the other areas, to quantify how much a type > safety change might help. Are most (all?) of the places working with SGLs? Maybe we could just have a 'get_user_pages_to_sgl' and 'put_pages_sgl' sort of interface that handled all this instead of trying to make something that is struct page based? It seems easier to get an extra bit for user/!user in the SGL datastructure? Jason
