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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>,
        Nikolay Borisov <nborisov@suse.com>
Subject: [PATCH v3 2/2] btrfs: allocate page arrays using bulk page allocator
Date:   Wed, 30 Mar 2022 16:11:23 -0400
Message-Id: <ede1d39f7878ee2ed12c1526cc2ec358a2d862cf.1648669832.git.sweettea-kernel@dorminy.me>
In-Reply-To: <cover.1648669832.git.sweettea-kernel@dorminy.me>
References: <cover.1648669832.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>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
---
Changes in v3:
 - Added a newline after variable declaration
Changes in v2:
 - Moved from ctree.c to extent_io.c
---
 fs/btrfs/extent_io.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index ab4c1c4d1b59..b268e47aa2b7 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3144,19 +3144,25 @@ static void end_bio_extent_readpage(struct bio *bio)
  */
 int btrfs_alloc_page_array(unsigned long nr_pages, struct page **page_array)
 {
-	int i;
+	long allocated = 0;
+
+	for (;;) {
+		long last = allocated;
 
-	for (i = 0; i < nr_pages; i++) {
-		struct page *page;
+		allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages,
+						   page_array);
+		if (allocated == nr_pages)
+			return 0;
 
-		if (page_array[i])
+		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