[PATCH] mm/gup: speed up check_and_migrate_cma_pages() on huge page

[Pingfan Liu <kernelfans@gmail.com>]

Both hugetlb and thp locate on the same migration type of pageblock, since
they are allocated from a free_list[]. Based on this fact, it is enough to
check on a single subpage to decide the migration type of the whole huge
page. By this way, it saves (2M/4K - 1) times loop for pmd_huge on x86,
similar on other archs.

Furthermore, when executing isolate_huge_page(), it avoid taking global
hugetlb_lock many times, and meanless remove/add to the local link list
cma_page_list.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Linux-kernel@vger.kernel.org
---
 mm/gup.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ddde097..2eecb16 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1342,16 +1342,19 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
 	LIST_HEAD(cma_page_list);
 
 check_again:
-	for (i = 0; i < nr_pages; i++) {
+	for (i = 0; i < nr_pages;) {
+
+		struct page *head = compound_head(pages[i]);
+		long step = 1;
+
+		if (PageCompound(head))
+			step = compound_order(head) - (pages[i] - head);
 		/*
 		 * If we get a page from the CMA zone, since we are going to
 		 * be pinning these entries, we might as well move them out
 		 * of the CMA zone if possible.
 		 */
 		if (is_migrate_cma_page(pages[i])) {
-
-			struct page *head = compound_head(pages[i]);
-
 			if (PageHuge(head)) {
 				isolate_huge_page(head, &cma_page_list);
 			} else {
@@ -1369,6 +1372,8 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
 				}
 			}
 		}
+
+		i += step;
 	}
 
 	if (!list_empty(&cma_page_list)) {
-- 
2.7.5