Re: [PATCH v11 01/25] mm/gup: factor out duplicate code from four routines

Kirill A. Shutemov
2019-12-18T22:45:16Z

On Wed, Dec 18, 2019 at 02:15:53PM -0800, John Hubbard wrote:
> On 12/18/19 7:52 AM, Kirill A. Shutemov wrote:
> > On Mon, Dec 16, 2019 at 02:25:13PM -0800, John Hubbard wrote:
> > > +static void put_compound_head(struct page *page, int refs)
> > > +{
> > > + /* Do a get_page() first, in case refs == page->_refcount */
> > > + get_page(page);
> > > + page_ref_sub(page, refs);
> > > + put_page(page);
> > > +}
> >
> > It's not terribly efficient. Maybe something like:
> >
> > VM_BUG_ON_PAGE(page_ref_count(page) < ref, page);
> > if (refs > 2)
> > page_ref_sub(page, refs - 1);
> > put_page(page);
> >
> > ?
>
> OK, but how about this instead? I don't see the need for a "2", as that
> is a magic number that requires explanation. Whereas "1" is not a magic
> number--here it means: either there are "many" (>1) refs, or not.

Yeah, it's my thinko. Sure, it has to be '1' (or >= 2, which is less readable).

> And the routine won't be called with refs less than about 32 (2MB huge
> page, 64KB base page == 32 subpages) anyway.

It's hard to make predictions about future :P

> VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
> /*
> * Calling put_page() for each ref is unnecessarily slow. Only the last
> * ref needs a put_page().
> */
> if (refs > 1)
> page_ref_sub(page, refs - 1);
> put_page(page);

Looks good to me.

--
Kirill A. Shutemov