Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Jerome Glisse]

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.

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 ?

Cheers,
Jérôme