[PATCH 5/5] mm/readahead: Use find_get_pages instead of radix_tree_lookup.

[raghu.prabhu13@gmail.com]

From: Raghavendra D Prabhu <rprabhu@wnohang.net>

Instead of running radix_tree_lookup in a loop and lock/unlocking in the
process, find_get_pages is called once, which returns a page_list, some of which
are not NULL and are in core.

Also, since find_get_pages returns number of pages, if all pages are already
cached, it can return early.

This will be mostly helpful when a higher proportion of nr_to_read pages are
already in the cache, which will mean less locking for page cache hits.

Signed-off-by: Raghavendra D Prabhu <rprabhu@wnohang.net>
---
 mm/readahead.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 3977455..3a1798d 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -157,35 +157,42 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 {
 	struct inode *inode = mapping->host;
 	struct page *page;
+	struct page **page_list = NULL;
 	unsigned long end_index;	/* The last page we want to read */
 	LIST_HEAD(page_pool);
 	int page_idx;
 	int ret = 0;
 	int ret_read = 0;
+	unsigned long num;
+	pgoff_t page_offset;
 	loff_t isize = i_size_read(inode);
 
 	if (isize == 0)
 		goto out;
 
+	page_list = kzalloc(nr_to_read * sizeof(struct page *), GFP_KERNEL);
+	if (!page_list)
+		goto out;
+
 	end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
+	num = find_get_pages(mapping, offset, nr_to_read, page_list);
 
 	/*
 	 * Preallocate as many pages as we will need.
 	 */
 	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
-		pgoff_t page_offset = offset + page_idx;
+		if (page_list[page_idx]) {
+			page_cache_release(page_list[page_idx]);
+			continue;
+		}
+
+		page_offset = offset + page_idx;
 
 		if (page_offset > end_index)
 			break;
 
-		rcu_read_lock();
-		page = radix_tree_lookup(&mapping->page_tree, page_offset);
-		rcu_read_unlock();
-		if (page)
-			continue;
-
 		page = page_cache_alloc_readahead(mapping);
-		if (!page)
+		if (unlikely(!page))
 			break;
 		page->index = page_offset;
 		list_add(&page->lru, &page_pool);
@@ -194,6 +201,13 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 			lookahead_size = 0;
 		}
 		ret++;
+
+		/*
+		 * Since num pages are already returned, bail out after
+		 * nr_to_read - num pages are allocated and added.
+		 */
+		if (ret == nr_to_read - num)
+			break;
 	}
 
 	/*
@@ -205,6 +219,7 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 		ret_read = read_pages(mapping, filp, &page_pool, ret);
 	BUG_ON(!list_empty(&page_pool));
 out:
+	kfree(page_list);
 	return (ret_read < 0 ? ret_read : ret);
 }
 
-- 
1.7.12.1

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