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From: John Hubbard <jhubbard@nvidia.com>
To: Jerome Glisse <jglisse@redhat.com>
Cc: 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: Mon, 28 Jan 2019 22:41:41 -0800
Message-ID: <3c3bb2a3-907b-819d-83ee-2b29802a5bda@nvidia.com> (raw)
In-Reply-To: <20190129012312.GB3359@redhat.com>

On 1/28/19 5:23 PM, Jerome Glisse wrote:
> On Mon, Jan 28, 2019 at 04:22:16PM -0800, John Hubbard wrote:
>> 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? :)
> 
> Correct, page_mkclean() does not check the refcount and do not need to
> check it. We need to check for the page pin after the page_mkclean when
> code is done prepping the page for io (clear_page_dirty_for_io).
> 
> The race Jan and I were discussing was about wether we needed to lock
> the page or not and we do not. For slow path page_mkclean and GUP_slow
> will synchronize on the page table lock. For GUP_fast the fast code will
> back off if the pte is not the same and thus either we see the pin after
> page_mkclean() or GUP_fast back off. You will never have code that miss
> the pin after page_mkclean() and GUP_fast that did not back off.

Here is the case I'm wondering about:

thread A                             thread B
--------                             --------
                                      gup_fast
page_mkclean
     is page gup-pinned?(no)
                                          page_cache_get_speculative
                                              (gup-pins the page here)
                                          check pte_val unchanged (yes)
        set_pte_at()

...and now thread A has created a read-only PTE, after gup_fast walked
the page tables and found a writeable entry. And so far, thread A has
not seen that the page is pinned.

What am I missing here? The above seems like a problem even before we
change anything.

> 
> Now the page_cache_get_speculative() is for another race when a page is
> freed concurrently. page_cache_get_speculative() only inc the refcount
> if the page is not already freed ie refcount != 0. So GUP_fast has 2
> exclusions mechanisms, one for racing modification to the page table
> like page_mkclean (pte the same after incrementing the refcount) and one
> for racing put_page (only increment refcount if it is not 0). Here for
> what we want we just modify this second mechanisms to add the bias
> value not just 1 to the refcount. This keep both mechanisms intacts
> and give us the page pin test through refcount bias value.
> 
> Note that page_mkclean can not race with a put_page() as whoever calls
> page_mkclean already hold a reference on the page and thus no put_page
> can free the page.
> 
> Does that help ?

Yes...getting close... :)

thanks,
-- 
John Hubbard
NVIDIA

  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
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 [this message]
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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