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From: John Hubbard <jhubbard@nvidia.com>
To: Jerome Glisse <jglisse@redhat.com>, Jan Kara <jack@suse.cz>
Cc: 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: Mon, 28 Jan 2019 16:22:16 -0800
Message-ID: <8492163b-8c50-6ea2-8bc9-8c445495ecb4@nvidia.com> (raw)
In-Reply-To: <20190123190409.GF3097@redhat.com>

On 1/23/19 11:04 AM, Jerome Glisse wrote:
> On Wed, Jan 23, 2019 at 07:02:30PM +0100, Jan Kara wrote:
>> On Tue 22-01-19 11:46:13, Jerome Glisse wrote:
>>> On Tue, Jan 22, 2019 at 04:24:59PM +0100, Jan Kara wrote:
>>>> On Thu 17-01-19 10:17:59, Jerome Glisse wrote:
>>>>> On Thu, Jan 17, 2019 at 10:30:47AM +0100, Jan Kara wrote:
>>>>>> On Wed 16-01-19 08:08:14, Jerome Glisse wrote:
>>>>>>> On Wed, Jan 16, 2019 at 12:38:19PM +0100, Jan Kara wrote:
>>>>>>>> On Tue 15-01-19 09:07:59, Jan Kara wrote:
>>>>>>>>> Agreed. So with page lock it would actually look like:
>>>>>>>>>
>>>>>>>>> get_page_pin()
>>>>>>>>> 	lock_page(page);
>>>>>>>>> 	wait_for_stable_page();
>>>>>>>>> 	atomic_add(&page->_refcount, PAGE_PIN_BIAS);
>>>>>>>>> 	unlock_page(page);
>>>>>>>>>
>>>>>>>>> And if we perform page_pinned() check under page lock, then if
>>>>>>>>> page_pinned() returned false, we are sure page is not and will not be
>>>>>>>>> pinned until we drop the page lock (and also until page writeback is
>>>>>>>>> completed if needed).
>>>>>>>>
>>>>>>>> After some more though, why do we even need wait_for_stable_page() and
>>>>>>>> lock_page() in get_page_pin()?
>>>>>>>>
>>>>>>>> During writepage page_mkclean() will write protect all page tables. So
>>>>>>>> there can be no new writeable GUP pins until we unlock the page as all such
>>>>>>>> GUPs will have to first go through fault and ->page_mkwrite() handler. And
>>>>>>>> that will wait on page lock and do wait_for_stable_page() for us anyway.
>>>>>>>> Am I just confused?
>>>>>>>
>>>>>>> Yeah with page lock it should synchronize on the pte but you still
>>>>>>> need to check for writeback iirc the page is unlocked after file
>>>>>>> system has queue up the write and thus the page can be unlock with
>>>>>>> write back pending (and PageWriteback() == trye) and i am not sure
>>>>>>> that in that states we can safely let anyone write to that page. I
>>>>>>> am assuming that in some case the block device also expect stable
>>>>>>> page content (RAID stuff).
>>>>>>>
>>>>>>> So the PageWriteback() test is not only for racing page_mkclean()/
>>>>>>> test_set_page_writeback() and GUP but also for pending write back.
>>>>>>
>>>>>> But this is prevented by wait_for_stable_page() that is already present in
>>>>>> ->page_mkwrite() handlers. Look:
>>>>>>
>>>>>> ->writepage()
>>>>>>   /* Page is locked here */
>>>>>>   clear_page_dirty_for_io(page)
>>>>>>     page_mkclean(page)
>>>>>>       -> page tables get writeprotected
>>>>>>     /* The following line will be added by our patches */
>>>>>>     if (page_pinned(page)) -> bounce
>>>>>>     TestClearPageDirty(page)
>>>>>>   set_page_writeback(page);
>>>>>>   unlock_page(page);
>>>>>>   ...submit_io...
>>>>>>
>>>>>> IRQ
>>>>>>   - IO completion
>>>>>>   end_page_writeback()
>>>>>>
>>>>>> So if GUP happens before page_mkclean() writeprotects corresponding PTE
>>>>>> (and these two actions are synchronized on the PTE lock), page_pinned()
>>>>>> will see the increment and report the page as pinned.
>>>>>>
>>>>>> If GUP happens after page_mkclean() writeprotects corresponding PTE, it
>>>>>> will fault:
>>>>>>   handle_mm_fault()
>>>>>>     do_wp_page()
>>>>>>       wp_page_shared()
>>>>>>         do_page_mkwrite()
>>>>>>           ->page_mkwrite() - that is block_page_mkwrite() or
>>>>>> 	    iomap_page_mkwrite() or whatever filesystem provides
>>>>>> 	  lock_page(page)
>>>>>>           ... prepare page ...
>>>>>> 	  wait_for_stable_page(page) -> this blocks until IO completes
>>>>>> 	    if someone cares about pages not being modified while under IO.
>>>>>
>>>>> The case i am worried is GUP see pte with write flag set but has not
>>>>> lock the page yet (GUP is get pte first, then pte to page then lock
>>>>> page), then it locks the page but the lock page can make it wait for a
>>>>> racing page_mkclean()...write back that have not yet write protected
>>>>> the pte the GUP just read. So by the time GUP has the page locked the
>>>>> pte it read might no longer have the write flag set. Hence why you need
>>>>> to also check for write back after taking the page lock. Alternatively
>>>>> you could recheck the pte after a successful try_lock on the page.
>>>>
>>>> This isn't really possible. GUP does:
>>>>
>>>> get_user_pages()
>>>> ...
>>>>   follow_page_mask()
>>>>   ...
>>>>     follow_page_pte()
>>>>       ptep = pte_offset_map_lock()
>>>>       check permissions and page sanity
>>>>       if (flags & FOLL_GET)
>>>>         get_page(page); -> this would become
>>>> 	  atomic_add(&page->_refcount, PAGE_PIN_BIAS);
>>>>       pte_unmap_unlock(ptep, ptl);
>>>>
>>>> page_mkclean() on the other hand grabs the same pte lock to change the pte
>>>> to write-protected. So after page_mkclean() has modified the PTE we are
>>>> racing on for access, we are sure to either see increased _refcount or get
>>>> page fault from GUP.
>>>>
>>>> If we see increased _refcount, we bounce the page and are fine. If GUP
>>>> faults, we will wait for page lock (so wait until page is prepared for IO
>>>> and has PageWriteback set) while handling the fault, then enter
>>>> ->page_mkwrite, which will do wait_for_stable_page() -> wait for
>>>> outstanding writeback to complete.
>>>>
>>>> So I still conclude - no need for page lock in the GUP path at all AFAICT.
>>>> In fact we rely on the very same page fault vs page writeback synchronization
>>>> for normal user faults as well. And normal user mmap access is even nastier
>>>> than GUP access because the CPU reads page tables without taking PTE lock.
>>>
>>> For the "slow" GUP path you are right you do not need a lock as the
>>> page table lock give you the ordering. For the GUP fast path you
>>> would either need the lock or the memory barrier with the test for
>>> page write back.
>>>
>>> Maybe an easier thing is to convert GUP fast to try to take the page
>>> table lock if it fails taking the page table lock then we fall back
>>> to slow GUP path. Otherwise then we have the same garantee as the slow
>>> path.
>>
>> You're right I was looking at the wrong place for GUP_fast() path. But I
>> still don't think anything special (i.e. page lock or new barrier) is
>> necessary. GUP_fast() takes care already now that it cannot race with page
>> unmapping or write-protection (as there are other places in MM that rely on
>> this). Look, gup_pte_range() has:
>>
>>                 if (!page_cache_get_speculative(head))
>>                         goto pte_unmap;
>>
>>                 if (unlikely(pte_val(pte) != pte_val(*ptep))) {
>>                         put_page(head);
>>                         goto pte_unmap;
>>                 }
>>
>> So that page_cache_get_speculative() will become
>> page_cache_pin_speculative() to increment refcount by PAGE_PIN_BIAS instead
>> of 1. That is atomic ordered operation so it cannot be reordered with the
>> following check that PTE stayed same. So once page_mkclean() write-protects
>> PTE, there can be no new pins from GUP_fast() and we are sure all
>> succeeding pins are visible in page->_refcount after page_mkclean()
>> completes. Again this is nothing new, other mm code already relies on
>> either seeing page->_refcount incremented or GUP fast bailing out (e.g. DAX
>> relies on this). Although strictly speaking I'm not 100% sure what prevents
>> page->_refcount load to be speculatively reordered before PTE update even
>> in current places using this but there's so much stuff inbetween that
>> there's probably something ;). But we could add smp_rmb() after
>> page_mkclean() before changing page_pinned() for the peace of mind I guess.
> 
> Yeah i think you are right, i missed the check on same pte value
> and the atomic inc in page_cache_get_speculative() is a barrier.
> I do not think the barrier would be necessary as page_mkclean is
> taking and dropping locks so those should have enough barriering.
> 

Hi Jan, Jerome,

OK, this seems to be up and running locally, but while putting together 
documentation and polishing up things, I noticed that there is one last piece 
that I don't quite understand, after all. The page_cache_get_speculative() 
existing documentation explains how refcount synchronizes these things, but I
don't see how that helps with synchronization page_mkclean and gup, in this 
situation:

    gup_fast gets the refcount and rechecks the pte hasn't changed

    meanwhile, page_mkclean...wait, how does refcount come into play here?
    page_mkclean can remove the mapping and insert a write-protected pte, 
    regardless of page refcount, correct?  Help? :)


thanks,
-- 
John Hubbard
NVIDIA

  reply index

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
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 [this message]
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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