On Wed, Dec 18, 2019 at 02:15:53PM -0800, John Hubbard wrote:

On 12/18/19 7:52 AM, Kirill A. Shutemov wrote:

quoted

On Mon, Dec 16, 2019 at 02:25:13PM -0800, John Hubbard wrote:

quoted

+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