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

[Jan Kara]

On Fri 14-12-18 11:38:59, Dan Williams wrote:
> On Thu, Dec 13, 2018 at 10:11 PM John Hubbard <jhubbard@xxxxxxxxxx> 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:
> 
> If we're going to allocate something new out-of-line then maybe we
> should go even further to allow for a page "proxy" object to front a
> real struct page. This idea arose from Dave Hansen as I explained to
> him the dax-reflink problem, and dovetails with Dave Chinner's
> suggestion earlier in this thread for dax-reflink.
> 
> Have get_user_pages() allocate a proxy object that gets passed around
> to drivers. Something like a struct page pointer with bit 0 set. This
> would add a conditional branch and pointer chase to many page
> operations, like page_to_pfn(), I thought something like it would be
> unacceptable a few years ago, but then HMM went and added similar
> overhead to put_page() and nobody balked.
> 
> This has the additional benefit of catching cases that might be doing
> a get_page() on a get_user_pages() result and should instead switch to
> a "ref_user_page()" (opposite of put_user_page()) as the API to take
> additional references on a get_user_pages() result.
> 
> page->index and page->mapping could be overridden by similar
> attributes in the proxy, and allow an N:1 relationship of proxy
> instances to actual pages. Filesystems could generate dynamic proxies
> as well.
> 
> The auxiliary information (dev_pagemap, hmm_data, etc...) moves to the
> proxy and stops polluting the base struct page which remains the
> canonical location for dirty-tracking and dma operations.
> 
> The difficulties are reconciling the source of the proxies as both
> get_user_pages() and filesystem may want to be the source of the
> allocation. In the get_user_pages_fast() path we may not be able to
> ask the filesystem for the proxy, at least not without destroying the
> performance expectations of get_user_pages_fast().

What you describe here sounds almost like page_ext mechanism we already
have? Or do you really aim at per-pin allocated structure?

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR