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

[Ira Weiny]

On Tue, Mar 19, 2019 at 04:36:44PM +0100, Jan Kara wrote:
> 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.

I'm glad to see this presented because it has crossed my mind more than once
that effectively a GUP pinned page should be considered "dirty" at all times
until the pin is removed.  This is especially true in the RDMA case.

> 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...

Agreed.  I continue to test with these patches and RDMA and have not seen any
problems thus far.

Ira

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