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From: Jerome Glisse <jglisse@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
	Matthew Wilcox <willy@infradead.org>,
	John Hubbard <john.hubbard@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux MM <linux-mm@kvack.org>, Jan Kara <jack@suse.cz>,
	tom@talpey.com, Al Viro <viro@zeniv.linux.org.uk>,
	benve@cisco.com, Christoph Hellwig <hch@infradead.org>,
	Christopher Lameter <cl@linux.com>,
	"Dalessandro, Dennis" <dennis.dalessandro@intel.com>,
	Doug Ledford <dledford@redhat.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, Michal Hocko <mhocko@kernel.org>,
	Mike Marciniszyn <mike.marciniszyn@intel.com>,
	rcampbell@nvidia.com,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
Date: Fri, 7 Dec 2018 14:40:09 -0500
Message-ID: <20181207194009.GF3293@redhat.com> (raw)
In-Reply-To: <CAPcyv4ig0Lg+6Abc+P9guOSdszk0B4LyHGh=wXf_kMzGs=5oww@mail.gmail.com>

On Fri, Dec 07, 2018 at 11:26:34AM -0800, Dan Williams wrote:
> On Fri, Dec 7, 2018 at 11:16 AM Jerome Glisse <jglisse@redhat.com> wrote:
> >
> > On Thu, Dec 06, 2018 at 06:45:49PM -0800, John Hubbard wrote:
> > > On 12/4/18 5:57 PM, John Hubbard wrote:
> > > > On 12/4/18 5:44 PM, Jerome Glisse wrote:
> > > >> On Tue, Dec 04, 2018 at 05:15:19PM -0800, Matthew Wilcox wrote:
> > > >>> On Tue, Dec 04, 2018 at 04:58:01PM -0800, John Hubbard wrote:
> > > >>>> On 12/4/18 3:03 PM, Dan Williams wrote:
> > > >>>>> Except the LRU fields are already in use for ZONE_DEVICE pages... how
> > > >>>>> does this proposal interact with those?
> > > >>>>
> > > >>>> Very badly: page->pgmap and page->hmm_data both get corrupted. Is there an entire
> > > >>>> use case I'm missing: calling get_user_pages() on ZONE_DEVICE pages? Said another
> > > >>>> way: is it reasonable to disallow calling get_user_pages() on ZONE_DEVICE pages?
> > > >>>>
> > > >>>> If we have to support get_user_pages() on ZONE_DEVICE pages, then the whole
> > > >>>> LRU field approach is unusable.
> > > >>>
> > > >>> We just need to rearrange ZONE_DEVICE pages.  Please excuse the whitespace
> > > >>> damage:
> > > >>>
> > > >>> +++ b/include/linux/mm_types.h
> > > >>> @@ -151,10 +151,12 @@ struct page {
> > > >>>  #endif
> > > >>>                 };
> > > >>>                 struct {        /* ZONE_DEVICE pages */
> > > >>> +                       unsigned long _zd_pad_2;        /* LRU */
> > > >>> +                       unsigned long _zd_pad_3;        /* LRU */
> > > >>> +                       unsigned long _zd_pad_1;        /* uses mapping */
> > > >>>                         /** @pgmap: Points to the hosting device page map. */
> > > >>>                         struct dev_pagemap *pgmap;
> > > >>>                         unsigned long hmm_data;
> > > >>> -                       unsigned long _zd_pad_1;        /* uses mapping */
> > > >>>                 };
> > > >>>
> > > >>>                 /** @rcu_head: You can use this to free a page by RCU. */
> > > >>>
> > > >>> You don't use page->private or page->index, do you Dan?
> > > >>
> > > >> page->private and page->index are use by HMM DEVICE page.
> > > >>
> > > >
> > > > OK, so for the ZONE_DEVICE + HMM case, that leaves just one field remaining for
> > > > dma-pinned information. Which might work. To recap, we need:
> > > >
> > > > -- 1 bit for PageDmaPinned
> > > > -- 1 bit, if using LRU field(s), for PageDmaPinnedWasLru.
> > > > -- N bits for a reference count
> > > >
> > > > Those *could* be packed into a single 64-bit field, if really necessary.
> > > >
> > >
> > > ...actually, this needs to work on 32-bit systems, as well. And HMM is using a lot.
> > > However, it is still possible for this to work.
> > >
> > > Matthew, can I have that bit now please? I'm about out of options, and now it will actually
> > > solve the problem here.
> > >
> > > Given:
> > >
> > > 1) It's cheap to know if a page is ZONE_DEVICE, and ZONE_DEVICE means not on the LRU.
> > > That, in turn, means only 1 bit instead of 2 bits (in addition to a counter) is required,
> > > for that case.
> > >
> > > 2) There is an independent bit available (according to Matthew).
> > >
> > > 3) HMM uses 4 of the 5 struct page fields, so only one field is available for a counter
> > >    in that case.
> >
> > To expend on this, HMM private page are use for anonymous page
> > so the index and mapping fields have the value you expect for
> > such pages. Down the road i want also to support file backed
> > page with HMM private (mapping, private, index).
> >
> > For HMM public both anonymous and file back page are supported
> > today (HMM public is only useful on platform with something like
> > OpenCAPI, CCIX or NVlink ... so PowerPC for now).
> >
> > > 4) get_user_pages() must work on ZONE_DEVICE and HMM pages.
> >
> > get_user_pages() only need to work with HMM public page not the
> > private one as we can not allow _anyone_ to pin HMM private page.
> 
> How does HMM enforce that? Because the kernel should not allow *any*
> memory management facility to arbitrarily fail direct-I/O operations.
> That's why CONFIG_FS_DAX_LIMITED is a temporary / experimental hack
> for S390 and ZONE_DEVICE was invented to bypass that hack for X86 and
> any arch that plans to properly support DAX. I would classify any
> memory management that can't support direct-I/O in the same
> "experimental" category.

It does not fail direct-I/O GUP sees a swap entry for the private
memory and it behave just like if the page was swap to disk so i
am not introducing any new behavior.

With HMM page everything just work as you expect they would from
CPU point of view. It is just like swap.

Cheers,
Jérôme

  reply index

Thread overview: 179+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-04  0:17 [PATCH 0/2] put_user_page*(): start converting the call sites john.hubbard
2018-12-04  0:17 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard
2018-12-04  7:53   ` Mike Rapoport
2018-12-05  1:40     ` John Hubbard
2018-12-04 20:28   ` Dan Williams
2018-12-04 21:56     ` John Hubbard
2018-12-04 23:03       ` Dan Williams
2018-12-05  0:36         ` Jerome Glisse
2018-12-05  0:40           ` Dan Williams
2018-12-05  0:59             ` John Hubbard
2018-12-05  0:58         ` John Hubbard
2018-12-05  1:00           ` Dan Williams
2018-12-05  1:15           ` Matthew Wilcox
2018-12-05  1:44             ` Jerome Glisse
2018-12-05  1:57               ` John Hubbard
2018-12-07  2:45                 ` John Hubbard
2018-12-07 19:16                   ` Jerome Glisse
2018-12-07 19:26                     ` Dan Williams
2018-12-07 19:40                       ` Jerome Glisse [this message]
2018-12-08  0:52                     ` John Hubbard
2018-12-08  2:24                       ` Jerome Glisse
2018-12-10 10:28                         ` Jan Kara
2018-12-12 15:03                           ` Jerome Glisse
2018-12-12 16:27                             ` Dan Williams
2018-12-12 17:02                               ` Jerome Glisse
2018-12-12 17:49                                 ` Dan Williams
2018-12-12 19:07                                   ` John Hubbard
2018-12-12 21:30                               ` Jerome Glisse
2018-12-12 21:40                                 ` Dan Williams
2018-12-12 21:53                                   ` Jerome Glisse
2018-12-12 22:11                                     ` Matthew Wilcox
2018-12-12 22:16                                       ` Jerome Glisse
2018-12-12 23:37                                     ` Jason Gunthorpe
2018-12-12 23:46                                       ` John Hubbard
2018-12-12 23:54                                       ` Dan Williams
2018-12-13  0:01                                       ` Jerome Glisse
2018-12-13  0:18                                         ` Dan Williams
2018-12-13  0:44                                           ` Jerome Glisse
2018-12-13  3:26                                             ` Jason Gunthorpe
2018-12-13  3:20                                         ` Jason Gunthorpe
2018-12-13 12:43                                           ` Jerome Glisse
2018-12-13 13:40                                             ` Tom Talpey
2018-12-13 14:18                                               ` Jerome Glisse
2018-12-13 14:51                                                 ` Tom Talpey
2018-12-13 15:18                                                   ` Jerome Glisse
2018-12-13 18:12                                                     ` Tom Talpey
2018-12-13 19:18                                                       ` Jerome Glisse
2018-12-14 10:41                                             ` Jan Kara
2018-12-14 15:25                                               ` Jerome Glisse
2018-12-12 21:56                                 ` John Hubbard
2018-12-12 22:04                                   ` Jerome Glisse
2018-12-12 22:11                                     ` John Hubbard
2018-12-12 22:14                                       ` Jerome Glisse
2018-12-12 22:17                                         ` John Hubbard
2018-12-12 21:46                             ` Dave Chinner
2018-12-12 21:59                               ` Jerome Glisse
2018-12-13  0:51                                 ` Dave Chinner
2018-12-13  2:02                                   ` Jerome Glisse
2018-12-13 15:56                                     ` Christopher Lameter
2018-12-13 16:02                                       ` Jerome Glisse
2018-12-14  6:00                                     ` Dave Chinner
2018-12-14 15:13                                       ` Jerome Glisse
2018-12-14  3:52                                   ` John Hubbard
2018-12-14  5:21                                     ` Dan Williams
2018-12-14  6:11                                       ` John Hubbard
2018-12-14 15:20                                         ` Jerome Glisse
2018-12-14 19:38                                         ` Dan Williams
2018-12-14 19:48                                           ` Matthew Wilcox
2018-12-14 19:53                                             ` Dave Hansen
2018-12-14 20:03                                               ` Matthew Wilcox
2018-12-14 20:17                                                 ` Dan Williams
2018-12-14 20:29                                                   ` Matthew Wilcox
2018-12-15  0:41                                                 ` John Hubbard
2018-12-17  8:56                                           ` Jan Kara
2018-12-17 18:28                                             ` Dan Williams
2018-12-14 15:43                               ` Jan Kara
2018-12-16 21:58                                 ` Dave Chinner
2018-12-17 18:11                                   ` Jerome Glisse
2018-12-17 18:34                                     ` Matthew Wilcox
2018-12-17 19:48                                       ` Jerome Glisse
2018-12-17 19:51                                         ` Matthew Wilcox
2018-12-17 19:54                                           ` Jerome Glisse
2018-12-17 19:59                                             ` Matthew Wilcox
2018-12-17 20:55                                               ` Jerome Glisse
2018-12-17 21:03                                                 ` Matthew Wilcox
2018-12-17 21:15                                                   ` Jerome Glisse
2018-12-18  1:09                                       ` Dave Chinner
2018-12-18  6:12                                       ` Darrick J. Wong
2018-12-18  9:30                                       ` Jan Kara
2018-12-18 23:29                                         ` John Hubbard
2018-12-19  2:07                                           ` Jerome Glisse
2018-12-19 11:08                                             ` Jan Kara
2018-12-20 10:54                                               ` John Hubbard
2018-12-20 16:50                                                 ` Jerome Glisse
2018-12-20 16:57                                                   ` Dan Williams
2018-12-20 16:49                                               ` Jerome Glisse
2019-01-03  1:55                                               ` Jerome Glisse
2019-01-03  3:27                                                 ` John Hubbard
2019-01-03 14:57                                                   ` Jerome Glisse
2019-01-03  9:26                                                 ` Jan Kara
2019-01-03 14:44                                                   ` Jerome Glisse
2019-01-11  2:59                                                     ` John Hubbard
2019-01-11 16:51                                                       ` Jerome Glisse
2019-01-12  1:04                                                         ` John Hubbard
2019-01-12  2:02                                                           ` Jerome Glisse
2019-01-12  2:38                                                             ` John Hubbard
2019-01-12  2:46                                                               ` Jerome Glisse
2019-01-12  3:06                                                                 ` John Hubbard
2019-01-12  3:25                                                                   ` Jerome Glisse
2019-01-12 20:46                                                                     ` John Hubbard
2019-01-14 14:54                                                                   ` Jan Kara
2019-01-14 17:21                                                                     ` Jerome Glisse
2019-01-14 19:09                                                                       ` John Hubbard
2019-01-15  8:34                                                                         ` Jan Kara
2019-01-15 21:39                                                                           ` John Hubbard
2019-01-15  8:07                                                                       ` Jan Kara
2019-01-15 17:15                                                                         ` Jerome Glisse
2019-01-15 21:56                                                                           ` John Hubbard
2019-01-15 22:12                                                                             ` Jerome Glisse
2019-01-16  0:44                                                                               ` John Hubbard
2019-01-16  1:56                                                                                 ` Jerome Glisse
2019-01-16  2:01                                                                                   ` Dan Williams
2019-01-16  2:23                                                                                     ` Jerome Glisse
2019-01-16  4:34                                                                                       ` Dave Chinner
2019-01-16 14:50                                                                                         ` Jerome Glisse
2019-01-16 22:51                                                                                           ` Dave Chinner
2019-01-16 11:38                                                                         ` Jan Kara
2019-01-16 13:08                                                                           ` Jerome Glisse
2019-01-17  5:42                                                                             ` John Hubbard
2019-01-17 15:21                                                                               ` Jerome Glisse
2019-01-18  0:16                                                                                 ` Dave Chinner
2019-01-18  1:59                                                                                   ` Jerome Glisse
2019-01-17  9:30                                                                             ` Jan Kara
2019-01-17 15:17                                                                               ` Jerome Glisse
2019-01-22 15:24                                                                                 ` Jan Kara
2019-01-22 16:46                                                                                   ` Jerome Glisse
2019-01-23 18:02                                                                                     ` Jan Kara
2019-01-23 19:04                                                                                       ` Jerome Glisse
2019-01-29  0:22                                                                                         ` John Hubbard
2019-01-29  1:23                                                                                           ` Jerome Glisse
2019-01-29  6:41                                                                                             ` John Hubbard
2019-01-29 10:12                                                                                               ` Jan Kara
2019-01-30  2:21                                                                                                 ` John Hubbard
2019-01-17  5:25                                                                         ` John Hubbard
2019-01-17  9:04                                                                           ` Jan Kara
2019-01-12  3:14                                                               ` Jerome Glisse
2018-12-18 10:33                                   ` Jan Kara
2018-12-18 23:42                                     ` Dave Chinner
2018-12-19  3:03                                       ` Jason Gunthorpe
2018-12-19  5:26                                         ` Dan Williams
2018-12-19 11:19                                           ` Jan Kara
2018-12-19 10:28                                         ` Dave Chinner
2018-12-19 11:35                                           ` Jan Kara
2018-12-19 16:56                                             ` Jason Gunthorpe
2018-12-19 22:33                                             ` Dave Chinner
2018-12-20  9:07                                               ` Jan Kara
2018-12-20 16:54                                               ` Jerome Glisse
2018-12-19 13:24                                       ` Jan Kara
2018-12-08  5:18                       ` Matthew Wilcox
2018-12-12 19:13                         ` John Hubbard
2018-12-08  7:16                       ` Dan Williams
2018-12-08 16:33                         ` Jerome Glisse
2018-12-08 16:48                           ` Christoph Hellwig
2018-12-08 17:47                             ` Jerome Glisse
2018-12-08 18:26                               ` Christoph Hellwig
2018-12-08 18:45                                 ` Jerome Glisse
2018-12-08 18:09                             ` Dan Williams
2018-12-08 18:12                               ` Christoph Hellwig
2018-12-11  6:18                               ` Dave Chinner
2018-12-05  5:52             ` Dan Williams
2018-12-05 11:16       ` Jan Kara
2018-12-04  0:17 ` [PATCH 2/2] infiniband/mm: convert put_page() to put_user_page*() john.hubbard
2018-12-04 17:10 ` [PATCH 0/2] put_user_page*(): start converting the call sites David Laight
2018-12-05  1:05   ` John Hubbard
2018-12-05 14:08     ` David Laight
2018-12-28  8:37       ` Pavel Machek
2019-02-08  7:56 [PATCH 0/2] mm: put_user_page() call site conversion first john.hubbard
2019-02-08  7:56 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard
2019-02-08 10:32   ` Mike Rapoport
2019-02-08 20:44     ` John Hubbard

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