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

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