Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Jerome Glisse]

On Thu, Dec 13, 2018 at 10:11:09PM -0800, John Hubbard wrote:
> On 12/13/18 9:21 PM, Dan Williams wrote:
> > On Thu, Dec 13, 2018 at 7:53 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
> >>
> >> On 12/12/18 4:51 PM, Dave Chinner wrote:
> >>> On Wed, Dec 12, 2018 at 04:59:31PM -0500, Jerome Glisse wrote:
> >>>> On Thu, Dec 13, 2018 at 08:46:41AM +1100, Dave Chinner wrote:
> >>>>> On Wed, Dec 12, 2018 at 10:03:20AM -0500, Jerome Glisse wrote:
> >>>>>> On Mon, Dec 10, 2018 at 11:28:46AM +0100, Jan Kara wrote:
> >>>>>>> On Fri 07-12-18 21:24:46, Jerome Glisse wrote:
> >>>>>>> So this approach doesn't look like a win to me over using counter in struct
> >>>>>>> page and I'd rather try looking into squeezing HMM public page usage of
> >>>>>>> struct page so that we can fit that gup counter there as well. I know that
> >>>>>>> it may be easier said than done...
> >>>>>>
> >>
> >> Agreed. After all the discussion this week, I'm thinking that the original idea
> >> of a per-struct-page counter is better. Fortunately, we can do the moral equivalent
> >> of that, unless I'm overlooking something: Jerome had another proposal that he
> >> described, off-list, for doing that counting, and his idea avoids the problem of
> >> finding space in struct page. (And in fact, when I responded yesterday, I initially
> >> thought that's where he was going with this.)
> >>
> >> So how about this hybrid solution:
> >>
> >> 1. Stay with the basic RFC approach of using a per-page counter, but actually
> >> store the counter(s) in the mappings instead of the struct page. We can use
> >> !PageAnon and page_mapping to look up all the mappings, stash the dma_pinned_count
> >> there. So the total pinned count is scattered across mappings. Probably still need
> >> a PageDmaPinned bit.
> > 
> > How do you safely look at page->mapping from the get_user_pages_fast()
> > path? You'll be racing invalidation disconnecting the page from the
> > mapping.
> > 
> 
> I don't have an answer for that, so maybe the page->mapping idea is dead already. 
> 
> So in that case, there is still one more way to do all of this, which is to
> combine ZONE_DEVICE, HMM, and gup/dma information in a per-page struct, and get
> there via basically page->private, more or less like this:

The page mapcount idea does work to get a pin count. So i believe
that this is what should be pursue, if no one wants to try it i
will do patches. Anything else is too invasive and requires too
much changes. Note that in all the discussion that happened in the
mapcount having a separate pin count would not have help one bit
nor would it solve the page_mkwrite issue.

So we need to audit put_user_page call place and see if they can
sleep and call mkwrite without issue. I believe the answer will be
yes for many ... maybe all.

Cheers,
Jérôme