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From:   Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
To:     Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Nick Terrell <terrelln@fb.com>, linux-kernel@vger.kernel.org,
        linux-btrfs@vger.kernel.org, kernel-team@fb.com
Cc:     Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Subject: [PATCH 2/2] btrfs: Allocate page arrays using bulk page allocator.
Date:   Mon, 28 Mar 2022 16:14:28 -0400
Message-Id: <9cbd861d3b302e19b990848ea747d2ea91d01aed.1648497027.git.sweettea-kernel@dorminy.me>
In-Reply-To: <cover.1648497027.git.sweettea-kernel@dorminy.me>
References: <cover.1648497027.git.sweettea-kernel@dorminy.me>
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Precedence: bulk

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>
---
 fs/btrfs/ctree.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 4e81e75c8e7c..37711a66e726 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -102,17 +102,24 @@ void btrfs_free_path(struct btrfs_path *p)
  */
 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