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

[Kirill A. Shutemov]

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.

Fair enough. Should we get it into a comment here?

> > > +void put_user_pages(struct page **pages, unsigned long npages)
> > > +{
> > > +	unsigned long index;
> > > +
> > > +	for (index = 0; index < npages; index++)
> > > +		put_user_page(pages[index]);
> > 
> > I believe there's an room for improvement for compound pages.
> > 
> > If there's multiple consequential pages in the array that belong to the
> > same compound page we can get away with a single atomic operation to
> > handle them all.
> 
> Yes maybe just add a comment with that for now and leave this kind of
> optimization to latter ?

Sounds good to me.

-- 
 Kirill A. Shutemov