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

[Jason Gunthorpe]

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