[PATCH RFC 7/9] mm/gup: Decrement head page once for group of subpages

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

Rather than decrementing the ref count one by one, we
walk the page array and checking which belong to the same
compound_head. Later on we decrement the calculated amount
of references in a single write to the head page.

Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
 mm/gup.c | 41 ++++++++++++++++++++++++++++++++---------
 1 file changed, 32 insertions(+), 9 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 194e6981eb03..3a9a7229f418 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -212,6 +212,18 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
 }
 #endif /* CONFIG_DEV_PAGEMAP_OPS */
 
+static int record_refs(struct page **pages, int npages)
+{
+	struct page *head = compound_head(pages[0]);
+	int refs = 1, index;
+
+	for (index = 1; index < npages; index++, refs++)
+		if (compound_head(pages[index]) != head)
+			break;
+
+	return refs;
+}
+
 /**
  * unpin_user_page() - release a dma-pinned page
  * @page:            pointer to page to be released
@@ -221,9 +233,9 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
  * that such pages can be separately tracked and uniquely handled. In
  * particular, interactions with RDMA and filesystems need special handling.
  */
-void unpin_user_page(struct page *page)
+static void __unpin_user_page(struct page *page, int refs)
 {
-	int refs = 1;
+	int orig_refs = refs;
 
 	page = compound_head(page);
 
@@ -237,14 +249,19 @@ void unpin_user_page(struct page *page)
 		return;
 
 	if (hpage_pincount_available(page))
-		hpage_pincount_sub(page, 1);
+		hpage_pincount_sub(page, refs);
 	else
-		refs = GUP_PIN_COUNTING_BIAS;
+		refs *= GUP_PIN_COUNTING_BIAS;
 
 	if (page_ref_sub_and_test(page, refs))
 		__put_page(page);
 
-	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
+	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, orig_refs);
+}
+
+void unpin_user_page(struct page *page)
+{
+	__unpin_user_page(page, 1);
 }
 EXPORT_SYMBOL(unpin_user_page);
 
@@ -274,6 +291,7 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 				 bool make_dirty)
 {
 	unsigned long index;
+	int refs = 1;
 
 	/*
 	 * TODO: this can be optimized for huge pages: if a series of pages is
@@ -286,8 +304,9 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 		return;
 	}
 
-	for (index = 0; index < npages; index++) {
+	for (index = 0; index < npages; index += refs) {
 		struct page *page = compound_head(pages[index]);
+
 		/*
 		 * Checking PageDirty at this point may race with
 		 * clear_page_dirty_for_io(), but that's OK. Two key
@@ -310,7 +329,8 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 		 */
 		if (!PageDirty(page))
 			set_page_dirty_lock(page);
-		unpin_user_page(page);
+		refs = record_refs(pages + index, npages - index);
+		__unpin_user_page(page, refs);
 	}
 }
 EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
@@ -327,6 +347,7 @@ EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
 void unpin_user_pages(struct page **pages, unsigned long npages)
 {
 	unsigned long index;
+	int refs = 1;
 
 	/*
 	 * If this WARN_ON() fires, then the system *might* be leaking pages (by
@@ -340,8 +361,10 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
 	 * physically contiguous and part of the same compound page, then a
 	 * single operation to the head page should suffice.
 	 */
-	for (index = 0; index < npages; index++)
-		unpin_user_page(pages[index]);
+	for (index = 0; index < npages; index += refs) {
+		refs = record_refs(pages + index, npages - index);
+		__unpin_user_page(pages[index], refs);
+	}
 }
 EXPORT_SYMBOL(unpin_user_pages);
 
-- 
2.17.1