[3/8] mm: Use a pagevec for readahead

Commit Message

Matthew Wilcox Jan. 13, 2020, 3:37 p.m. UTC

From: "Matthew Wilcox (Oracle)" <[e-mail]>

Instead of using a linked list, use a small array.  This does mean we
will allocate and then submit for I/O no more than 15 pages at a time
(60kB), but we have the block queue plugged so the bios can be combined
afterwards.  We generally don't readahead more than 256kB anyway,
so this is not a huge reduction in efficiency, and we'll make up
for it with later patches.

Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
---
 mm/readahead.c | 97 +++++++++++++++++++++++++++-----------------------
 1 file changed, 52 insertions(+), 45 deletions(-)

Patch

diff --git a/mm/readahead.c b/mm/readahead.c
index 6bf73ef33b7e..76a70a4406b5 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -113,35 +113,37 @@  int read_cache_pages(struct address_space *mapping, struct list_head *pages,
 
 EXPORT_SYMBOL(read_cache_pages);
 
-static int read_pages(struct address_space *mapping, struct file *filp,
-		struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
+/*
+ * We ignore I/O errors - they will be handled by the actual consumer of
+ * the data that we attempted to prefetch.
+ */
+static unsigned read_pages(struct address_space *mapping, struct file *filp,
+		struct pagevec *pvec, pgoff_t offset, gfp_t gfp)
 {
-	struct blk_plug plug;
-	unsigned page_idx;
-	int ret;
-
-	blk_start_plug(&plug);
+	struct page *page;
+	unsigned int nr_pages = pagevec_count(pvec);
 
 	if (mapping->a_ops->readpages) {
-		ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
-		/* Clean up the remaining pages */
-		put_pages_list(pages);
-		goto out;
-	}
+		LIST_HEAD(pages);
 
-	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
-		struct page *page = lru_to_page(pages);
-		list_del(&page->lru);
-		if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
-			mapping->a_ops->readpage(filp, page);
-		put_page(page);
+		pagevec_for_each(pvec, page) {
+			page->index = offset++;
+			list_add(&page->lru, &pages);
+		}
+		mapping->a_ops->readpages(filp, mapping, &pages, nr_pages);
+		/* Clean up the remaining pages */
+		put_pages_list(&pages);
+	} else {
+		pagevec_for_each(pvec, page) {
+			if (!add_to_page_cache_lru(page, mapping, offset++,
+						gfp))
+				mapping->a_ops->readpage(filp, page);
+			put_page(page);
+		}
 	}
-	ret = 0;
 
-out:
-	blk_finish_plug(&plug);
-
-	return ret;
+	pagevec_reinit(pvec);
+	return nr_pages;
 }
 
 /*
@@ -159,59 +161,64 @@  unsigned long __do_page_cache_readahead(struct address_space *mapping,
 	struct inode *inode = mapping->host;
 	struct page *page;
 	unsigned long end_index;	/* The last page we want to read */
-	LIST_HEAD(page_pool);
+	struct pagevec pages;
 	int page_idx;
+	pgoff_t page_offset = offset;
 	unsigned long nr_pages = 0;
 	loff_t isize = i_size_read(inode);
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
+	struct blk_plug plug;
+
+	blk_start_plug(&plug);
 
 	if (isize == 0)
 		goto out;
 
 	end_index = ((isize - 1) >> PAGE_SHIFT);
+	pagevec_init(&pages);
 
 	/*
 	 * 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;
+		page_offset++;
 
 		if (page_offset > end_index)
 			break;
 
 		page = xa_load(&mapping->i_pages, page_offset);
+
+		/*
+		 * Page already present?  Kick off the current batch of
+		 * contiguous pages before continuing with the next batch.
+		 */
 		if (page && !xa_is_value(page)) {
-			/*
-			 * Page already present?  Kick off the current batch of
-			 * contiguous pages before continuing with the next
-			 * batch.
-			 */
-			if (nr_pages)
-				read_pages(mapping, filp, &page_pool, nr_pages,
-						gfp_mask);
-			nr_pages = 0;
+			unsigned int count = pagevec_count(&pages);
+
+			if (count)
+				nr_pages += read_pages(mapping, filp, &pages,
+						offset, gfp_mask);
+			offset = page_offset + 1;
 			continue;
 		}
 
 		page = __page_cache_alloc(gfp_mask);
 		if (!page)
 			break;
-		page->index = page_offset;
-		list_add(&page->lru, &page_pool);
+		if (pagevec_add(&pages, page) == 0) {
+			nr_pages += read_pages(mapping, filp, &pages,
+					offset, gfp_mask);
+			offset = page_offset + 1;
+		}
 		if (page_idx == nr_to_read - lookahead_size)
 			SetPageReadahead(page);
-		nr_pages++;
 	}
 
-	/*
-	 * Now start the IO.  We ignore I/O errors - if the page is not
-	 * uptodate then the caller will launch readpage again, and
-	 * will then handle the error.
-	 */
-	if (nr_pages)
-		read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
-	BUG_ON(!list_empty(&page_pool));
+	if (pagevec_count(&pages))
+		nr_pages += read_pages(mapping, filp, &pages, offset, gfp_mask);
 out:
+	blk_finish_plug(&plug);
+
 	return nr_pages;
 }