Re: [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder versions

[Jason Gunthorpe]

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