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

[Jerome Glisse]

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@xxxxxxxxx wrote:
> > > > > From: John Hubbard <jhubbard@xxxxxxxxxx>
> > > 
> > > [...]
> > > 
> > > > > 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. The plan is multi-step
(with different patchset for each steps):
    [1] convert all places that do gup() and then put_page() to
        use gup_put_page() instead. This is what this present
        patchset is about.
    [2] use bias pin count so that we can identify GUPed page from
        non GUPed page. So instead of incrementing page refcount
        by 1 on GUP we increment it by GUP_BIAS and in gup_put_page()
        we decrement it by GUP_BIAS. This means that page with a
        refcount > GUP_BIAS can be considered as GUP (more on false
        positive below)
    [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.

For race between clear_page_for_io() and GUP this was extensively
discuss and IIRC you were on that thread. Basicly we can only race
with page_mkclean() (as GUP can only succeed if there is a pte with
write permission) and page cleaning happens after page_mkclean() and
they are barrier between page_mkclean() and what's after. Hence we
will be able to see the page as GUPed before cleaning it without
any race.

For false positive i think we agreed that it is something we can
live with. It could only happen to page that are share more than
GUP_BIAS times, it should be rare enough and false positive means
you get the same treatement as a GUPed page.

Cheers,
Jérôme