Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions

[Jerome Glisse <jglisse@redhat.com>]

On Wed, Mar 20, 2019 at 10:57:52AM +1100, Dave Chinner wrote:
> On Tue, Mar 19, 2019 at 06:06:55PM -0400, Jerome Glisse wrote:
> > On Wed, Mar 20, 2019 at 08:23:46AM +1100, Dave Chinner wrote:
> > > On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote:
> > > > On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote:
> > > > > On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote:
> > > > > > On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@gmail.com wrote:
> > > > > > > From: John Hubbard <jhubbard@nvidia.com>
> > > > > 
> > > > > [...]
> > > > > 
> > > > > > > diff --git a/mm/gup.c b/mm/gup.c
> > > > > > > index f84e22685aaa..37085b8163b1 100644
> > > > > > > --- a/mm/gup.c
> > > > > > > +++ b/mm/gup.c
> > > > > > > @@ -28,6 +28,88 @@ struct follow_page_context {
> > > > > > >  	unsigned int page_mask;
> > > > > > >  };
> > > > > > >  
> > > > > > > +typedef int (*set_dirty_func_t)(struct page *page);
> > > > > > > +
> > > > > > > +static void __put_user_pages_dirty(struct page **pages,
> > > > > > > +				   unsigned long npages,
> > > > > > > +				   set_dirty_func_t sdf)
> > > > > > > +{
> > > > > > > +	unsigned long index;
> > > > > > > +
> > > > > > > +	for (index = 0; index < npages; index++) {
> > > > > > > +		struct page *page = compound_head(pages[index]);
> > > > > > > +
> > > > > > > +		if (!PageDirty(page))
> > > > > > > +			sdf(page);
> > > > > > 
> > > > > > How is this safe? What prevents the page to be cleared under you?
> > > > > > 
> > > > > > If it's safe to race clear_page_dirty*() it has to be stated explicitly
> > > > > > with a reason why. It's not very clear to me as it is.
> > > > > 
> > > > > The PageDirty() optimization above is fine to race with clear the
> > > > > page flag as it means it is racing after a page_mkclean() and the
> > > > > GUP user is done with the page so page is about to be write back
> > > > > ie if (!PageDirty(page)) see the page as dirty and skip the sdf()
> > > > > call while a split second after TestClearPageDirty() happens then
> > > > > it means the racing clear is about to write back the page so all
> > > > > is fine (the page was dirty and it is being clear for write back).
> > > > > 
> > > > > If it does call the sdf() while racing with write back then we
> > > > > just redirtied the page just like clear_page_dirty_for_io() would
> > > > > do if page_mkclean() failed so nothing harmful will come of that
> > > > > neither. Page stays dirty despite write back it just means that
> > > > > the page might be write back twice in a row.
> > > > 
> > > > Forgot to mention one thing, we had a discussion with Andrea and Jan
> > > > about set_page_dirty() and Andrea had the good idea of maybe doing
> > > > the set_page_dirty() at GUP time (when GUP with write) not when the
> > > > GUP user calls put_page(). We can do that by setting the dirty bit
> > > > in the pte for instance. They are few bonus of doing things that way:
> > > >     - amortize the cost of calling set_page_dirty() (ie one call for
> > > >       GUP and page_mkclean()
> > > >     - it is always safe to do so at GUP time (ie the pte has write
> > > >       permission and thus the page is in correct state)
> > > >     - safe from truncate race
> > > >     - no need to ever lock the page
> > > 
> > > I seem to have missed this conversation, so please excuse me for
> > 
> > The set_page_dirty() at GUP was in a private discussion (it started
> > on another topic and drifted away to set_page_dirty()).
> > 
> > > asking a stupid question: if it's a file backed page, what prevents
> > > background writeback from cleaning the dirty page ~30s into a long
> > > term pin? i.e. I don't see anything in this proposal that prevents
> > > the page from being cleaned by writeback and putting us straight
> > > back into the situation where a long term RDMA is writing to a clean
> > > page....
> > 
> > So this patchset does not solve this issue.
> 
> OK, so it just kicks the can further down the road.
> 
> >     [3..N] decide what to do for GUPed page, so far the plans seems
> >          to be to keep the page always dirty and never allow page
> >          write back to restore the page in a clean state. This does
> >          disable thing like COW and other fs feature but at least
> >          it seems to be the best thing we can do.
> 
> So the plan for GUP vs writeback so far is "break fsync()"? :)
> 
> We might need to work on that a bit more...

Sorry forgot to say that we still do write back using a bounce page
so that at least we write something to disk that is just a snapshot
of the GUPed page everytime writeback kicks in (so either through
radix tree dirty page write back or fsync or any other sync events).
So many little details that i forgot the big chunk :)

Cheers,
Jérôme