[PATCH 3/4] mm/gup: add a range variant of unpin_user_pages_dirty_lock()

[Joao Martins]

Add a unpin_user_page_range() API which takes a starting page
and how many consecutive pages we want to dirty.

Given that we won't be iterating on a list of changes, change
compound_next() to receive a bool, whether to calculate from the starting
page, or walk the page array. Finally add a separate iterator,
for_each_compound_range() that just operate in page ranges as opposed
to page array.

For users (like RDMA mr_dereg) where each sg represents a
contiguous set of pages, we're able to more efficiently unpin
pages without having to supply an array of pages much of what
happens today with unpin_user_pages().

Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
 include/linux/mm.h |  2 ++
 mm/gup.c           | 48 ++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a608feb0d42e..b76063f7f18a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1265,6 +1265,8 @@ static inline void put_page(struct page *page)
 void unpin_user_page(struct page *page);
 void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 				 bool make_dirty);
+void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
+				      bool make_dirty);
 void unpin_user_pages(struct page **pages, unsigned long npages);
 
 /**
diff --git a/mm/gup.c b/mm/gup.c
index 971a24b4b73f..1b57355d5033 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -215,11 +215,16 @@ void unpin_user_page(struct page *page)
 }
 EXPORT_SYMBOL(unpin_user_page);
 
-static inline unsigned int count_ntails(struct page **pages, unsigned long npages)
+static inline unsigned int count_ntails(struct page **pages,
+					unsigned long npages, bool range)
 {
-	struct page *head = compound_head(pages[0]);
+	struct page *page = pages[0], *head = compound_head(page);
 	unsigned int ntails;
 
+	if (range)
+		return (!PageCompound(head) || compound_order(head) <= 1) ? 1 :
+		   min_t(unsigned int, (head + compound_nr(head) - page), npages);
+
 	for (ntails = 1; ntails < npages; ntails++) {
 		if (compound_head(pages[ntails]) != head)
 			break;
@@ -229,20 +234,32 @@ static inline unsigned int count_ntails(struct page **pages, unsigned long npage
 }
 
 static inline void compound_next(unsigned long i, unsigned long npages,
-				 struct page **list, struct page **head,
-				 unsigned int *ntails)
+				 struct page **list, bool range,
+				 struct page **head, unsigned int *ntails)
 {
+	struct page *p, **next = &p;
+
 	if (i >= npages)
 		return;
 
-	*ntails = count_ntails(list + i, npages - i);
-	*head = compound_head(list[i]);
+	if (range)
+		*next = *list + i;
+	else
+		next = list + i;
+
+	*ntails = count_ntails(next, npages - i, range);
+	*head = compound_head(*next);
 }
 
+#define for_each_compound_range(i, list, npages, head, ntails) \
+	for (i = 0, compound_next(i, npages, list, true, &head, &ntails); \
+	     i < npages; i += ntails, \
+	     compound_next(i, npages, list, true,  &head, &ntails))
+
 #define for_each_compound_head(i, list, npages, head, ntails) \
-	for (i = 0, compound_next(i, npages, list, &head, &ntails); \
+	for (i = 0, compound_next(i, npages, list, false, &head, &ntails); \
 	     i < npages; i += ntails, \
-	     compound_next(i, npages, list, &head, &ntails))
+	     compound_next(i, npages, list, false,  &head, &ntails))
 
 /**
  * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
@@ -306,6 +323,21 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 }
 EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
 
+void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
+				      bool make_dirty)
+{
+	unsigned long index;
+	struct page *head;
+	unsigned int ntails;
+
+	for_each_compound_range(index, &page, npages, head, ntails) {
+		if (make_dirty && !PageDirty(head))
+			set_page_dirty_lock(head);
+		put_compound_head(head, ntails, FOLL_PIN);
+	}
+}
+EXPORT_SYMBOL(unpin_user_page_range_dirty_lock);
+
 /**
  * unpin_user_pages() - release an array of gup-pinned pages.
  * @pages:  array of pages to be marked dirty and released.
-- 
2.17.1