[PATCH v2 2/2] btrfs: allocate page arrays using bulk page allocator

[Sweet Tea Dorminy <sweettea-kernel@dorminy.me>]

While calling alloc_page() in a loop is an effective way to populate an
array of pages, the kernel provides a method to allocate pages in bulk.
alloc_pages_bulk_array() populates the NULL slots in a page array, trying to
grab more than one page at a time.

Unfortunately, it doesn't guarantee allocating all slots in the array,
but it's easy to call it in a loop and return an error if no progress
occurs. Similar code can be found in xfs/xfs_buf.c:xfs_buf_alloc_pages().

Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
---
Changes in v2:
 - Moved from ctree.c to extent_io.c
---
 fs/btrfs/extent_io.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 5700fcc23271..52abad421ba1 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3144,17 +3144,24 @@ static void end_bio_extent_readpage(struct bio *bio)
  */
 int btrfs_alloc_page_array(unsigned long nr_pages, struct page **page_array)
 {
-	int i;
-	for (i = 0; i < nr_pages; i++) {
-		struct page *page;
-		if (page_array[i])
+	long allocated = 0;
+	for (;;) {
+		long last = allocated;
+
+		allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages,
+						   page_array);
+		if (allocated == nr_pages)
+			return 0;
+
+		if (allocated != last)
 			continue;
-		page = alloc_page(GFP_NOFS);
-		if (!page)
-			return -ENOMEM;
-		page_array[i] = page;
+		/*
+		 * During this iteration, no page could be allocated, even
+		 * though alloc_pages_bulk_array() falls back to alloc_page()
+		 * if  it could not bulk-allocate. So we must be out of memory.
+		 */
+		return -ENOMEM;
 	}
-	return 0;
 }
 
 /*
-- 
2.35.1