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 [thread overview]
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
next prev parent reply other threads:[~2019-01-29 6:41 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
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 [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 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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