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

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);

?