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Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
To:     Jerome Glisse <jglisse@redhat.com>, Jan Kara <jack@suse.cz>
CC:     Matthew Wilcox <willy@infradead.org>,
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From:   John Hubbard <jhubbard@nvidia.com>
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On 1/3/19 6:44 AM, Jerome Glisse wrote:
> On Thu, Jan 03, 2019 at 10:26:54AM +0100, Jan Kara wrote:
>> On Wed 02-01-19 20:55:33, Jerome Glisse wrote:
>>> On Wed, Dec 19, 2018 at 12:08:56PM +0100, Jan Kara wrote:
>>>> 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?
>>>>
>>>
>>> Ok so i found a solution for that. First GUP must wait for racing
>>> write back. If GUP see a valid write-able PTE and the page has
>>> write back flag set then it must back of as if the PTE was not
>>> valid to force fault. It is just a race with page_mkclean and we
>>> want ordering between the two. Note this is not strictly needed
>>> so we can relax that but i believe this ordering is better to do
>>> in GUP rather then having each single user of GUP test for this
>>> to avoid the race.
>>>
>>> GUP increase mapcount only after checking that it is not racing
>>> with writeback it also set a page flag (SetPageDMAPined(page)).
>>>
>>> When clearing a write-able pte we set a special entry inside the
>>> page table (might need a new special swap type for this) and change
>>> page_mkclean_one() to clear to 0 those special entry.
>>>
>>>
>>> Now page_mkclean:
>>>
>>> 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,
>>>     };
>>> +   int mapcount1, mapcount2;
>>>
>>>     BUG_ON(!PageLocked(page));
>>>
>>>     if (!page_mapped(page))
>>>         return 0;
>>>
>>>     mapping = page_mapping(page);
>>>     if (!mapping)
>>>         return 0;
>>>
>>> +   mapcount1 = page_mapcount(page);
>>>     // rmap_walk need to change to count mapping and return value
>>>     // in .mapcount easy one
>>>     rmap_walk(page, &rwc);
>>
>> So what prevents GUP_fast() to grab reference here and the test below would
>> think the page is not pinned? Or do you assume that every page_mkclean()
>> call will be protected by PageWriteback (currently it is not) so that
>> GUP_fast() blocks / bails out?

Continuing this thread, still focusing only on the "how to maintain a PageDmaPinned
for each page" question (ignoring, for now, what to actually *do* in response to 
that flag being set):

1. Jan's point above is still a problem: PageWriteback != "page_mkclean is happening".
This is probably less troubling than the next point, but it does undermine all the 
complicated schemes involving PageWriteback, that try to synchronize gup() with
page_mkclean().

2. Also, the mapcount approach here still does not reliably avoid false negatives
(that is, a page may have been gup'd, but page_mkclean could miss that): gup()
can always jump in and increment the mapcount, while page_mkclean is in the middle
of making (wrong) decisions based on that mapcount. There's no lock to prevent that.

Again: mapcount can go up *or* down, so I'm not seeing a true solution yet.

> 
> So GUP_fast() becomes:
> 
> GUP_fast_existing() { ... }
> GUP_fast()
> {
>     GUP_fast_existing();
> 
>     for (i = 0; i < npages; ++i) {
>         if (PageWriteback(pages[i])) {
>             // need to force slow path for this page
>         } else {
>             SetPageDmaPinned(pages[i]);
>             atomic_inc(pages[i]->mapcount);
>         }
>     }
> }
> 
> This is a minor slow down for GUP fast and it takes care of a
> write back race on behalf of caller. This means that page_mkclean
> can not see a mapcount value that increase. This simplify thing
> we can relax that. Note that what this is doing is making sure
> that GUP_fast never get lucky :) ie never GUP a page that is in
> the process of being write back but has not yet had its pte
> updated to reflect that.
> 
> 
>> But I think that detecting pinned pages with small false positive rate is
>> OK. The extra page bouncing will cost some performance but if it is rare,
>> then we are OK. So I think we can go for the simple version of detecting
>> pinned pages as you mentioned in some earlier email. We just have to be
>> sure there are no false negatives.
> 

Agree with that sentiment, but there are still false negatives and I'm not
yet seeing any solutions for that.

thanks,
-- 
John Hubbard
NVIDIA