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

[John Hubbard]

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.

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

	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);


thanks,
--
John Hubbard
NVIDIA