From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: MIME-Version: 1.0 References: <20181205014441.GA3045@redhat.com> <59ca5c4b-fd5b-1fc6-f891-c7986d91908e@nvidia.com> <7b4733be-13d3-c790-ff1b-ac51b505e9a6@nvidia.com> <20181207191620.GD3293@redhat.com> <3c4d46c0-aced-f96f-1bf3-725d02f11b60@nvidia.com> <20181208022445.GA7024@redhat.com> <20181210102846.GC29289@quack2.suse.cz> <20181212150319.GA3432@redhat.com> <20181212214641.GB29416@dastard> <20181212215931.GG5037@redhat.com> <20181213005119.GD29416@dastard> <05a68829-6e6d-b766-11b4-99e1ba4bc87b@nvidia.com> <01cf4e0c-b2d6-225a-3ee9-ef0f7e53684d@nvidia.com> In-Reply-To: <01cf4e0c-b2d6-225a-3ee9-ef0f7e53684d@nvidia.com> From: Dan Williams Date: Fri, 14 Dec 2018 11:38:59 -0800 Message-ID: Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions To: John Hubbard Cc: david , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Jan Kara , Matthew Wilcox , John Hubbard , Andrew Morton , Linux MM , tom@talpey.com, Al Viro , benve@cisco.com, Christoph Hellwig , Christopher Lameter , "Dalessandro, Dennis" , Doug Ledford , Jason Gunthorpe , Michal Hocko , Mike Marciniszyn , rcampbell@nvidia.com, Linux Kernel Mailing List , linux-fsdevel , Dave Hansen Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: On Thu, Dec 13, 2018 at 10:11 PM John Hubbard wrote: > > On 12/13/18 9:21 PM, Dan Williams wrote: > > On Thu, Dec 13, 2018 at 7:53 PM John Hubbard 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(). > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 5ed8f6292a53..13f651bb5cc1 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -67,6 +67,13 @@ struct hmm; > #define _struct_page_alignment > #endif > > +struct page_aux { > + struct dev_pagemap *pgmap; > + unsigned long hmm_data; > + unsigned long private; > + atomic_t dma_pinned_count; > +}; > + > struct page { > unsigned long flags; /* Atomic flags, some possibly > * updated asynchronously */ > @@ -149,11 +156,13 @@ struct page { > spinlock_t ptl; > #endif > }; > - struct { /* ZONE_DEVICE pages */ > + struct { /* ZONE_DEVICE, HMM or get_user_pages() pages */ > /** @pgmap: Points to the hosting device page map. */ > - struct dev_pagemap *pgmap; > - unsigned long hmm_data; > - unsigned long _zd_pad_1; /* uses mapping */ > + unsigned long _zd_pad_1; /* LRU */ > + unsigned long _zd_pad_2; /* LRU */ > + unsigned long _zd_pad_3; /* mapping */ > + unsigned long _zd_pad_4; /* index */ > + struct page_aux *aux; /* private */ > }; > > /** @rcu_head: You can use this to free a page by RCU. */ > > ...is there any appetite for that approach? > > -- > thanks, > John Hubbard > NVIDIA