[PATCH v2 1/4] mm/gup: add compound page list iterator

[Joao Martins]

Add an helper that iterates over head pages in a list of pages. It
essentially counts the tails until the next page to process has a
different head that the current. This is going to be used by
unpin_user_pages() family of functions, to batch the head page refcount
updates once for all passed consecutive tail pages.

Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
 mm/gup.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index d68bcb482b11..d1549c61c2f6 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -215,6 +215,35 @@ void unpin_user_page(struct page *page)
 }
 EXPORT_SYMBOL(unpin_user_page);
 
+static inline void compound_next(unsigned long i, unsigned long npages,
+				 struct page **list, struct page **head,
+				 unsigned int *ntails)
+{
+	struct page *page;
+	unsigned int nr;
+
+	if (i >= npages)
+		return;
+
+	list += i;
+	npages -= i;
+	page = compound_head(*list);
+
+	for (nr = 1; nr < npages; nr++) {
+		if (compound_head(list[nr]) != page)
+			break;
+	}
+
+	*head = page;
+	*ntails = nr;
+}
+
+#define for_each_compound_head(__i, __list, __npages, __head, __ntails) \
+	for (__i = 0, \
+	     compound_next(__i, __npages, __list, &(__head), &(__ntails)); \
+	     __i < __npages; __i += __ntails, \
+	     compound_next(__i, __npages, __list, &(__head), &(__ntails)))
+
 /**
  * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
  * @pages:  array of pages to be maybe marked dirty, and definitely released.
-- 
2.17.1