[PATCH v2 3/4] mm/gup: add a range variant of unpin_user_pages_dirty_lock()

[Joao Martins]

Add a unpin_user_page_range_dirty_lock() API which takes a starting page
and how many consecutive pages we want to unpin and optionally 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           | 64 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

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 5a3dd235017a..3426736a01b2 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -215,6 +215,34 @@ void unpin_user_page(struct page *page)
 }
 EXPORT_SYMBOL(unpin_user_page);
 
+static inline void range_next(unsigned long i, unsigned long npages,
+			      struct page **list, struct page **head,
+			      unsigned int *ntails)
+{
+	struct page *next, *page;
+	unsigned int nr = 1;
+
+	if (i >= npages)
+		return;
+
+	npages -= i;
+	next = *list + i;
+
+	page = compound_head(next);
+	if (PageCompound(page) && compound_order(page) > 1)
+		nr = min_t(unsigned int,
+			   page + compound_nr(page) - next, npages);
+
+	*head = page;
+	*ntails = nr;
+}
+
+#define for_each_compound_range(__i, __list, __npages, __head, __ntails) \
+	for (__i = 0, \
+	     range_next(__i, __npages, __list, &(__head), &(__ntails)); \
+	     __i < __npages; __i += __ntails, \
+	     range_next(__i, __npages, __list, &(__head), &(__ntails)))
+
 static inline void compound_next(unsigned long i, unsigned long npages,
 				 struct page **list, struct page **head,
 				 unsigned int *ntails)
@@ -306,6 +334,42 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 }
 EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
 
+/**
+ * unpin_user_page_range_dirty_lock() - release and optionally dirty
+ * gup-pinned page range
+ *
+ * @page:  the starting page of a range maybe marked dirty, and definitely released.
+ * @npages: number of consecutive pages to release.
+ * @make_dirty: whether to mark the pages dirty
+ *
+ * "gup-pinned page range" refers to a range of pages that has had one of the
+ * get_user_pages() variants called on that page.
+ *
+ * For the page ranges defined by [page .. page+npages], make that range (or
+ * its head pages, if a compound page) dirty, if @make_dirty is true, and if the
+ * page range was previously listed as clean.
+ *
+ * set_page_dirty_lock() is used internally. If instead, set_page_dirty() is
+ * required, then the caller should a) verify that this is really correct,
+ * because _lock() is usually required, and b) hand code it:
+ * set_page_dirty_lock(), unpin_user_page().
+ *
+ */
+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