[PATCH v6 07/19] mm: Put readahead pages in cache earlier

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>

At allocation time, put the pages in the cache unless we're using
->readpages.  Add the readahead_for_each() iterator for the benefit of
the ->readpage fallback.  This iterator supports huge pages, even though
none of the filesystems to be converted do yet.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 include/linux/pagemap.h | 24 ++++++++++++++++++++++++
 mm/readahead.c          | 34 +++++++++++++++++-----------------
 2 files changed, 41 insertions(+), 17 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 982ecda2d4a2..3613154e79e4 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -639,8 +639,32 @@ struct readahead_control {
 /* private: use the readahead_* accessors instead */
 	pgoff_t _start;
 	unsigned int _nr_pages;
+	unsigned int _batch_count;
 };
 
+static inline struct page *readahead_page(struct readahead_control *rac)
+{
+	struct page *page;
+
+	if (!rac->_nr_pages)
+		return NULL;
+
+	page = xa_load(&rac->mapping->i_pages, rac->_start);
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+	rac->_batch_count = hpage_nr_pages(page);
+
+	return page;
+}
+
+static inline void readahead_next(struct readahead_control *rac)
+{
+	rac->_nr_pages -= rac->_batch_count;
+	rac->_start += rac->_batch_count;
+}
+
+#define readahead_for_each(rac, page)					\
+	for (; (page = readahead_page(rac)); readahead_next(rac))
+
 /* The number of pages in this readahead block */
 static inline unsigned int readahead_count(struct readahead_control *rac)
 {
diff --git a/mm/readahead.c b/mm/readahead.c
index bdc5759000d3..9e430daae42f 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -113,12 +113,11 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
 
 EXPORT_SYMBOL(read_cache_pages);
 
-static void read_pages(struct readahead_control *rac, struct list_head *pages,
-		gfp_t gfp)
+static void read_pages(struct readahead_control *rac, struct list_head *pages)
 {
 	const struct address_space_operations *aops = rac->mapping->a_ops;
+	struct page *page;
 	struct blk_plug plug;
-	unsigned page_idx;
 
 	blk_start_plug(&plug);
 
@@ -127,19 +126,13 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
 				readahead_count(rac));
 		/* Clean up the remaining pages */
 		put_pages_list(pages);
-		goto out;
-	}
-
-	for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
-		struct page *page = lru_to_page(pages);
-		list_del(&page->lru);
-		if (!add_to_page_cache_lru(page, rac->mapping, page->index,
-				gfp))
+	} else {
+		readahead_for_each(rac, page) {
 			aops->readpage(rac->file, page);
-		put_page(page);
+			put_page(page);
+		}
 	}
 
-out:
 	blk_finish_plug(&plug);
 }
 
@@ -159,6 +152,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
 	unsigned long i;
 	loff_t isize = i_size_read(inode);
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
+	bool use_list = mapping->a_ops->readpages;
 	struct readahead_control rac = {
 		.mapping = mapping,
 		.file = filp,
@@ -196,8 +190,14 @@ void __do_page_cache_readahead(struct address_space *mapping,
 		page = __page_cache_alloc(gfp_mask);
 		if (!page)
 			break;
-		page->index = offset;
-		list_add(&page->lru, &page_pool);
+		if (use_list) {
+			page->index = offset;
+			list_add(&page->lru, &page_pool);
+		} else if (add_to_page_cache_lru(page, mapping, offset,
+					gfp_mask) < 0) {
+			put_page(page);
+			goto read;
+		}
 		if (i == nr_to_read - lookahead_size)
 			SetPageReadahead(page);
 		rac._nr_pages++;
@@ -205,7 +205,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
 		continue;
 read:
 		if (readahead_count(&rac))
-			read_pages(&rac, &page_pool, gfp_mask);
+			read_pages(&rac, &page_pool);
 		rac._nr_pages = 0;
 		rac._start = ++offset;
 	}
@@ -216,7 +216,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
 	 * will then handle the error.
 	 */
 	if (readahead_count(&rac))
-		read_pages(&rac, &page_pool, gfp_mask);
+		read_pages(&rac, &page_pool);
 	BUG_ON(!list_empty(&page_pool));
 }
 
-- 
2.25.0