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

[Jan Kara]

On Tue 19-03-19 17:29:18, Kirill A. Shutemov 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
> > 
> > Extra bonus from my point of view, it simplify thing for my generic
> > page protection patchset (KSM for file back page).
> > 
> > So maybe we should explore that ? It would also be a lot less code.
> 
> Yes, please. It sounds more sensible to me to dirty the page on get, not
> on put.

I fully agree this is a desirable final state of affairs. And with changes
to how we treat pinned pages during writeback there won't have to be any
explicit dirtying at all in the end because the page is guaranteed to be
dirty after a write page fault and pin would make sure it stays dirty until
unpinned. However initially I want the helpers to be as close to code they
are replacing as possible. Because it will be hard to catch all the bugs
due to driver conversions even in that situation. So I still think that
these helpers as they are a good first step. Then we need to convert
GUP users to use them and then it is much easier to modify the behavior
since it is no longer opencoded in two hudred or how many places...

								Honza

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR