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

[Jan Kara <jack@suse.cz>]

On Mon 14-01-19 12:21:25, Jerome Glisse wrote:
> On Mon, Jan 14, 2019 at 03:54:47PM +0100, Jan Kara wrote:
> > On Fri 11-01-19 19:06:08, John Hubbard wrote:
> > > On 1/11/19 6:46 PM, Jerome Glisse wrote:
> > > > On Fri, Jan 11, 2019 at 06:38:44PM -0800, John Hubbard wrote:
> > > > [...]
> > > > 
> > > >>>> The other idea that you and Dan (and maybe others) pointed out was a debug
> > > >>>> option, which we'll certainly need in order to safely convert all the call
> > > >>>> sites. (Mirror the mappings at a different kernel offset, so that put_page()
> > > >>>> and put_user_page() can verify that the right call was made.)  That will be
> > > >>>> a separate patchset, as you recommended.
> > > >>>>
> > > >>>> I'll even go as far as recommending the page lock itself. I realize that this 
> > > >>>> adds overhead to gup(), but we *must* hold off page_mkclean(), and I believe
> > > >>>> that this (below) has similar overhead to the notes above--but is *much* easier
> > > >>>> to verify correct. (If the page lock is unacceptable due to being so widely used,
> > > >>>> then I'd recommend using another page bit to do the same thing.)
> > > >>>
> > > >>> Please page lock is pointless and it will not work for GUP fast. The above
> > > >>> scheme do work and is fine. I spend the day again thinking about all memory
> > > >>> ordering and i do not see any issues.
> > > >>>
> > > >>
> > > >> Why is it that page lock cannot be used for gup fast, btw?
> > > > 
> > > > Well it can not happen within the preempt disable section. But after
> > > > as a post pass before GUP_fast return and after reenabling preempt then
> > > > it is fine like it would be for regular GUP. But locking page for GUP
> > > > is also likely to slow down some workload (with direct-IO).
> > > > 
> > > 
> > > Right, and so to crux of the matter: taking an uncontended page lock
> > > involves pretty much the same set of operations that your approach does.
> > > (If gup ends up contended with the page lock for other reasons than these
> > > paths, that seems surprising.) I'd expect very similar performance.
> > > 
> > > But the page lock approach leads to really dramatically simpler code (and
> > > code reviews, let's not forget). Any objection to my going that
> > > direction, and keeping this idea as a Plan B? I think the next step will
> > > be, once again, to gather some performance metrics, so maybe that will
> > > help us decide.
> > 
> > FWIW I agree that using page lock for protecting page pinning (and thus
> > avoid races with page_mkclean()) looks simpler to me as well and I'm not
> > convinced there will be measurable difference to the more complex scheme
> > with barriers Jerome suggests unless that page lock contended. Jerome is
> > right that you cannot just do lock_page() in gup_fast() path. There you
> > have to do trylock_page() and if that fails just bail out to the slow gup
> > path.
> > 
> > Regarding places other than page_mkclean() that need to check pinned state:
> > Definitely page migration will want to check whether the page is pinned or
> > not so that it can deal differently with short-term page references vs
> > longer-term pins.
> > 
> > Also there is one more idea I had how to record number of pins in the page:
> > 
> > #define PAGE_PIN_BIAS	1024
> > 
> > get_page_pin()
> > 	atomic_add(&page->_refcount, PAGE_PIN_BIAS);
> > 
> > put_page_pin();
> > 	atomic_add(&page->_refcount, -PAGE_PIN_BIAS);
> > 
> > page_pinned(page)
> > 	(atomic_read(&page->_refcount) - page_mapcount(page)) > PAGE_PIN_BIAS
> > 
> > This is pretty trivial scheme. It still gives us 22-bits for page pins
> > which should be plenty (but we should check for that and bail with error if
> > it would overflow). Also there will be no false negatives and false
> > positives only if there are more than 1024 non-page-table references to the
> > page which I expect to be rare (we might want to also subtract
> > hpage_nr_pages() for radix tree references to avoid excessive false
> > positives for huge pages although at this point I don't think they would
> > matter). Thoughts?
> 
> Racing PUP are as likely to cause issues:
> 
> CPU0                        | CPU1       | CPU2
>                             |            |
>                             | PUP()      |
>     page_pinned(page)       |            |
>       (page_count(page) -   |            |
>        page_mapcount(page)) |            |
>                             |            | GUP()
> 
> So here the refcount snap-shot does not include the second GUP and
> we can have a false negative ie the page_pinned() will return false
> because of the PUP happening just before on CPU1 despite the racing
> GUP on CPU2 just after.
> 
> I believe only either lock or memory ordering with barrier can
> guarantee that we do not miss GUP ie no false negative. Still the
> bias idea might be usefull as with it we should not need a flag.

Right. We need similar synchronization (i.e., page lock or careful checks
with memory barriers) if we want to get a reliable page pin information.

> So to make the above safe it would still need the page write back
> double check that i described so that GUP back-off if it raced with
> page_mkclean,clear_page_dirty_for_io and the fs write page call back
> which call test_set_page_writeback() (yes it is very unlikely but
> might still happen).

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).

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