From: Jan Kara <jack@suse.cz>
To: Jerome Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
Matthew Wilcox <willy@infradead.org>,
Dave Chinner <david@fromorbit.com>,
Dan Williams <dan.j.williams@intel.com>,
John Hubbard <john.hubbard@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Linux MM <linux-mm@kvack.org>,
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@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: Wed, 19 Dec 2018 12:08:56 +0100 [thread overview]
Message-ID: <20181219110856.GA18345@quack2.suse.cz> (raw)
In-Reply-To: <20181219020723.GD4347@redhat.com>
On Tue 18-12-18 21:07:24, Jerome Glisse wrote:
> On Tue, Dec 18, 2018 at 03:29:34PM -0800, John Hubbard wrote:
> > OK, so let's take another look at Jerome's _mapcount idea all by itself (using
> > *only* the tracking pinned pages aspect), given that it is the lightest weight
> > solution for that.
> >
> > So as I understand it, this would use page->_mapcount to store both the real
> > mapcount, and the dma pinned count (simply added together), but only do so for
> > file-backed (non-anonymous) pages:
> >
> >
> > __get_user_pages()
> > {
> > ...
> > get_page(page);
> >
> > if (!PageAnon)
> > atomic_inc(page->_mapcount);
> > ...
> > }
> >
> > put_user_page(struct page *page)
> > {
> > ...
> > if (!PageAnon)
> > atomic_dec(&page->_mapcount);
> >
> > put_page(page);
> > ...
> > }
> >
> > ...and then in the various consumers of the DMA pinned count, we use page_mapped(page)
> > to see if any mapcount remains, and if so, we treat it as DMA pinned. Is that what you
> > had in mind?
>
> Mostly, with the extra two observations:
> [1] We only need to know the pin count when a write back kicks in
> [2] We need to protect GUP code with wait_for_write_back() in case
> GUP is racing with a write back that might not the see the
> elevated mapcount in time.
>
> So for [2]
>
> __get_user_pages()
> {
> get_page(page);
>
> if (!PageAnon) {
> atomic_inc(page->_mapcount);
> + if (PageWriteback(page)) {
> + // Assume we are racing and curent write back will not see
> + // the elevated mapcount so wait for current write back and
> + // force page fault
> + wait_on_page_writeback(page);
> + // force slow path that will fault again
> + }
> }
> }
This is not needed AFAICT. __get_user_pages() gets page reference (and it
should also increment page->_mapcount) under PTE lock. So at that point we
are sure we have writeable PTE nobody can change. So page_mkclean() has to
block on PTE lock to make PTE read-only and only after going through all
PTEs like this, it can check page->_mapcount. So the PTE lock provides
enough synchronization.
> For [1] only needing pin count during write back turns page_mkclean into
> the perfect spot to check for that so:
>
> int page_mkclean(struct page *page)
> {
> int cleaned = 0;
> + int real_mapcount = 0;
> struct address_space *mapping;
> struct rmap_walk_control rwc = {
> .arg = (void *)&cleaned,
> .rmap_one = page_mkclean_one,
> .invalid_vma = invalid_mkclean_vma,
> + .mapcount = &real_mapcount,
> };
>
> BUG_ON(!PageLocked(page));
>
> if (!page_mapped(page))
> return 0;
>
> mapping = page_mapping(page);
> if (!mapping)
> return 0;
>
> // rmap_walk need to change to count mapping and return value
> // in .mapcount easy one
> rmap_walk(page, &rwc);
>
> // Big fat comment to explain what is going on
> + if ((page_mapcount(page) - real_mapcount) > 0) {
> + SetPageDMAPined(page);
> + } else {
> + ClearPageDMAPined(page);
> + }
This is the detail I'm not sure about: Why cannot rmap_walk_file() race
with e.g. zap_pte_range() which decrements page->_mapcount and thus the
check we do in page_mkclean() is wrong?
>
> // Maybe we want to leverage the int nature of return value so that
> // we can express more than cleaned/truncated and express cleaned/
> // truncated/pinned for benefit of caller and that way we do not
> // even need one bit as page flags above.
>
> return cleaned;
> }
>
> You do not want to change page_mapped() i do not see a need for that.
>
> Then the whole discussion between Jan and Dave seems to indicate that
> the bounce mechanism will need to be in the fs layer and that we can
> not reuse the bio bounce mechanism. This means that more work is needed
> at the fs level for that (so that fs do not freak on bounce page).
>
> Note that they are few gotcha where we need to preserve the pin count
> ie mostly in truncate code path that can remove page from page cache
> and overwrite the mapcount in the process, this would need to be fixed
> to not overwrite mapcount so that put_user_page does not set the map
> count to an invalid value turning the page into a bad state that will
> at one point trigger kernel BUG_ON();
>
> I am not saying block truncate, i am saying make sure it does not
> erase pin count and keep truncating happily. The how to handle truncate
> is a per existing GUP user discussion to see what they want to do for
> that.
>
> Obviously a bit deeper analysis of all spot that use mapcount is needed
> to check that we are not breaking anything but from the top of my head
> i can not think of anything bad (migrate will abort and other things will
> assume the page is mapped even it is only in hardware page table, ...).
Hum, grepping for page_mapped() and page_mapcount(), this is actually going
to be non-trivial to get right AFAICT.
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2018-12-19 11:08 UTC|newest]
Thread overview: 213+ 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
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 [this message]
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 2:59 ` John Hubbard
2019-01-11 16:51 ` Jerome Glisse
2019-01-11 16:51 ` Jerome Glisse
2019-01-12 1:04 ` John Hubbard
2019-01-12 1:04 ` John Hubbard
2019-01-12 2:02 ` Jerome Glisse
2019-01-12 2:02 ` Jerome Glisse
2019-01-12 2:38 ` John Hubbard
2019-01-12 2:38 ` John Hubbard
2019-01-12 2:46 ` Jerome Glisse
2019-01-12 2:46 ` Jerome Glisse
2019-01-12 3:06 ` John Hubbard
2019-01-12 3:06 ` John Hubbard
2019-01-12 3:25 ` Jerome Glisse
2019-01-12 3:25 ` Jerome Glisse
2019-01-12 20:46 ` John Hubbard
2019-01-12 20:46 ` John Hubbard
2019-01-14 14:54 ` Jan Kara
2019-01-14 14:54 ` Jan Kara
2019-01-14 17:21 ` Jerome Glisse
2019-01-14 17:21 ` Jerome Glisse
2019-01-14 19:09 ` John Hubbard
2019-01-14 19:09 ` John Hubbard
2019-01-15 8:34 ` Jan Kara
2019-01-15 8:34 ` Jan Kara
2019-01-15 21:39 ` John Hubbard
2019-01-15 21:39 ` John Hubbard
2019-01-15 8:07 ` Jan Kara
2019-01-15 8:07 ` Jan Kara
2019-01-15 17:15 ` Jerome Glisse
2019-01-15 17:15 ` Jerome Glisse
2019-01-15 21:56 ` John Hubbard
2019-01-15 21:56 ` John Hubbard
2019-01-15 22:12 ` Jerome Glisse
2019-01-15 22:12 ` Jerome Glisse
2019-01-16 0:44 ` John Hubbard
2019-01-16 0:44 ` John Hubbard
2019-01-16 1:56 ` Jerome Glisse
2019-01-16 1:56 ` Jerome Glisse
2019-01-16 2:01 ` Dan Williams
2019-01-16 2:01 ` Dan Williams
2019-01-16 2:23 ` Jerome Glisse
2019-01-16 2:23 ` Jerome Glisse
2019-01-16 4:34 ` Dave Chinner
2019-01-16 4:34 ` Dave Chinner
2019-01-16 14:50 ` Jerome Glisse
2019-01-16 14:50 ` Jerome Glisse
2019-01-16 22:51 ` Dave Chinner
2019-01-16 22:51 ` Dave Chinner
2019-01-16 11:38 ` Jan Kara
2019-01-16 11:38 ` Jan Kara
2019-01-16 13:08 ` Jerome Glisse
2019-01-16 13:08 ` Jerome Glisse
2019-01-17 5:42 ` John Hubbard
2019-01-17 5:42 ` John Hubbard
2019-01-17 15:21 ` Jerome Glisse
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 9:30 ` Jan Kara
2019-01-17 15:17 ` Jerome Glisse
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 5:25 ` John Hubbard
2019-01-17 9:04 ` Jan Kara
2019-01-17 9:04 ` Jan Kara
2019-01-12 3:14 ` Jerome Glisse
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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