On Sun 04-11-18 23:17:58, John Hubbard wrote: > On 10/22/18 12:43 PM, Jason Gunthorpe wrote: > > 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? > > I finally put together a spreadsheet, in order to answer this sort of thing. > Some notes: > > a) There are around 100 call sites of either get_user_pages*(), or indirect > calls via iov_iter_get_pages*(). Quite a bit... > b) There are only a few SGL users. Most are ad-hoc, instead: some loop that > either can be collapsed nicely into the new put_user_pages*() APIs, or... > cannot. > > c) The real problem is: around 20+ iov_iter_get_pages*() call sites. I need > to change both the iov_iter system a little bit, and also change the callers > so that they don't pile all the gup-pinned pages into the same page** array > that also contains other allocation types. This can be done, it just takes > time, that's the good news. Yes, but looking into iov_iter_get_pages() users, lot of them then end up feeding the result either in SGL, SKB (which is basically the same thing, just for networking), or BVEC (which is again a very similar thing, just for generic block layer). I'm not saying that we must have _sgl() interface as untangling all those users might be just too complex but there is certainly some space for unification and common interfaces ;) Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
