On Thu 11-10-18 20:53:34, John Hubbard wrote: > On 10/11/18 6:23 PM, 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. > > > > Back to page flags again, out of desperation: > > > > How much do we know about the page types that all of these subsystems > > use? In other words, can we, for example, use bit 1 of page->lru.next (see [1] > > for context) as the "dma-pinned" page flag, while tracking pages within parts > > of the kernel that call a mix of alloc_pages, get_user_pages, and other allocators? > > In order for that to work, page->index, page->private, and bit 1 of page->mapping > > must not be used. I doubt that this is always going to hold, but...does it? > > > > Oops, pardon me, please ignore that nonsense about page->index and page->private > and page->mapping, that's actually fine (I was seeing "union", where "struct" was > written--too much staring at this code). > > So actually, I think maybe we can just use bit 1 in page->lru.next to sort out > which pages are dma-pinned, in the calling code, just like we're going to do > in writeback situations. This should also allow run-time checking that Andrew was > hoping for: > > put_user_page(): assert that the page is dma-pinned > put_page(): assert that the page is *not* dma-pinned > > ...both of which depend on that bit being, essentially, available as sort > of a general page flag. And in fact, if it's not, then the whole approach > is dead anyway. Well, put_page() cannot assert page is not dma-pinned as someone can still to get_page(), put_page() on dma-pinned page and that must not barf. But put_page() could assert that if the page is pinned, refcount is >= pincount. That will detect leaked pin references relatively quickly. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
