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

>
> // Maybe we want to leverage the int nature of return value so that
> // we can express more than cleaned/truncated and express cleaned/
> // truncated/pinned for benefit of caller and that way we do not
> // even need one bit as page flags above.
>
> return cleaned;
> }
>
> You do not want to change page_mapped() i do not see a need for that.
>
> Then the whole discussion between Jan and Dave seems to indicate that
> the bounce mechanism will need to be in the fs layer and that we can
> not reuse the bio bounce mechanism. This means that more work is needed
> at the fs level for that (so that fs do not freak on bounce page).
>
> Note that they are few gotcha where we need to preserve the pin count
> ie mostly in truncate code path that can remove page from page cache
> and overwrite the mapcount in the process, this would need to be fixed
> to not overwrite mapcount so that put_user_page does not set the map
> count to an invalid value turning the page into a bad state that will
> at one point trigger kernel BUG_ON();
>
> I am not saying block truncate, i am saying make sure it does not
> erase pin count and keep truncating happily. The how to handle truncate
> is a per existing GUP user discussion to see what they want to do for
> that.
>
> Obviously a bit deeper analysis of all spot that use mapcount is needed
> to check that we are not breaking anything but from the top of my head
> i can not think of anything bad (migrate will abort and other things will
> assume the page is mapped even it is only in hardware page table, ...).

Hum, grepping for page_mapped() and page_mapcount(), this is actually going
to be non-trivial to get right AFAICT.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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