linux-kernel - [PATCH 6.1.y v2 5/6] btrfs: reuse cloned extent buffer during fiemap to avoid re-allocations

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Message-Id: <20251114194438.5694-6-kalachev@swemel.ru>
Date: Fri, 14 Nov 2025 22:44:37 +0300
From: Andrey Kalachev <kalachev@...mel.ru>
To: linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	fdmanana@...e.com,
	josef@...icpanda.com,
	dsterba@...e.com
Cc: stable@...r.kernel.org,
	kalachev@...mel.ru,
	lvc-project@...uxtesting.org
Subject: [PATCH 6.1.y v2 5/6] btrfs: reuse cloned extent buffer during fiemap to avoid re-allocations

From: Filipe Manana <fdmanana@...e.com>

[ Upstream commit 1cab1375ba6d5337a25acb346996106c12bb2dd0 ]

During fiemap we may have to visit multiple leaves of the subvolume's
inode tree, and each time we are freeing and allocating an extent buffer
to use as a clone of each visited leaf. Optimize this by reusing cloned
extent buffers, to avoid the freeing and re-allocation both of the extent
buffer structure itself and more importantly of the pages attached to the
extent buffer.

CC: stable@...r.kernel.org # 6.1
Reviewed-by: Josef Bacik <josef@...icpanda.com>
Signed-off-by: Filipe Manana <fdmanana@...e.com>
Signed-off-by: David Sterba <dsterba@...e.com>
[cherry picked from upstream commit]
Signed-off-by: Andrey Kalachev <kalachev@...mel.ru>
---
 fs/btrfs/extent_io.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index b65cd7adb7e3..da90e5a5f855 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3800,7 +3800,7 @@ static int emit_last_fiemap_cache(struct fiemap_extent_info *fieinfo,
 
 static int fiemap_next_leaf_item(struct btrfs_inode *inode, struct btrfs_path *path)
 {
-	struct extent_buffer *clone;
+	struct extent_buffer *clone = path->nodes[0];
 	struct btrfs_key key;
 	int slot;
 	int ret;
@@ -3809,29 +3809,45 @@ static int fiemap_next_leaf_item(struct btrfs_inode *inode, struct btrfs_path *p
 	if (path->slots[0] < btrfs_header_nritems(path->nodes[0]))
 		return 0;
 
+	/*
+	 * Add a temporary extra ref to an already cloned extent buffer to
+	 * prevent btrfs_next_leaf() freeing it, we want to reuse it to avoid
+	 * the cost of allocating a new one.
+	 */
+	ASSERT(test_bit(EXTENT_BUFFER_UNMAPPED, &clone->bflags));
+	atomic_inc(&clone->refs);
+
 	ret = btrfs_next_leaf(inode->root, path);
 	if (ret != 0)
-		return ret;
+		goto out;
 
 	/*
 	 * Don't bother with cloning if there are no more file extent items for
 	 * our inode.
 	 */
 	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
-	if (key.objectid != btrfs_ino(inode) || key.type != BTRFS_EXTENT_DATA_KEY)
-		return 1;
+	if (key.objectid != btrfs_ino(inode) || key.type != BTRFS_EXTENT_DATA_KEY) {
+		ret = 1;
+		goto out;
+	}
 
 	/* See the comment at fiemap_search_slot() about why we clone. */
-	clone = btrfs_clone_extent_buffer(path->nodes[0]);
-	if (!clone)
-		return -ENOMEM;
+	copy_extent_buffer_full(clone, path->nodes[0]);
+	/*
+	 * Important to preserve the start field, for the optimizations when
+	 * checking if extents are shared (see extent_fiemap()).
+	 */
+	clone->start = path->nodes[0]->start;
 
 	slot = path->slots[0];
 	btrfs_release_path(path);
 	path->nodes[0] = clone;
 	path->slots[0] = slot;
+out:
+	if (ret)
+		free_extent_buffer(clone);
 
-	return 0;
+	return ret;
 }
 
 /*
-- 
2.39.5