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

[John Hubbard <jhubbard@nvidia.com>]

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*().

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.

> 
> 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?
> 

So at the moment I don't think we need this *_sgl interface. We need iov_iter*
changes instead.

thanks,
-- 
John Hubbard
NVIDIA