[PATCH v2 3/8] mm: Optimise madvise WILLNEED

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Instead of calling find_get_entry() for every page index, use an XArray
iterator to skip over NULL entries, and avoid calling get_page(),
because we only want the swap entries.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/madvise.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index dd1d43cf026d..96189acd6969 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -224,25 +224,28 @@ static void force_shm_swapin_readahead(struct vm_area_struct *vma,
 		unsigned long start, unsigned long end,
 		struct address_space *mapping)
 {
-	pgoff_t index;
+	XA_STATE(xas, &mapping->i_pages, linear_page_index(vma, start));
+	pgoff_t end_index = end / PAGE_SIZE;
 	struct page *page;
-	swp_entry_t swap;
 
-	for (; start < end; start += PAGE_SIZE) {
-		index = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
+	rcu_read_lock();
+	xas_for_each(&xas, page, end_index) {
+		swp_entry_t swap;
 
-		page = find_get_entry(mapping, index);
-		if (!xa_is_value(page)) {
-			if (page)
-				put_page(page);
+		if (!xa_is_value(page))
 			continue;
-		}
+		rcu_read_unlock();
+
 		swap = radix_to_swp_entry(page);
 		page = read_swap_cache_async(swap, GFP_HIGHUSER_MOVABLE,
 							NULL, 0, false);
 		if (page)
 			put_page(page);
+
+		rcu_read_lock();
+		xas_reset(&xas);
 	}
+	rcu_read_unlock();
 
 	lru_add_drain();	/* Push any new pages onto the LRU now */
 }
-- 
2.28.0