[PATCH v2 2/3] btrfs: early exit the searching process in search_tree ioctl

[Sun YangKai <sunk67188@gmail.com>]

With the following two assumptions(min_key and max_key here is the
related fields in search_key):

1. When `btrfs_search_forward` returns 0, the key will always greater
   than or equal to the provided key. So the function `copy_to_sk` will
   never be called with a key less than the min_key. So we will never get
   a key less than min_key during walking forward in the leaf.

2. When we got a key greater than max_key in current leaf, we will never
   get a key in search_key range any more. So we should stop searching
   further.

So we can safely replace the continue with early exiting. This will
improve the performance of the search_tree related ioctl syscalls, which
is the bottleneck of tools like compsize.

I've run compsize with and without this patch to compare the performance.

When cache is cold, it's
1.29user 21.02system 0:34.61elapsed 64%CPU without the patch, and
1.24user 17.34system 0:30.40elapsed 61%CPU with the patch.

When cache is hot, it's
1.21user 12.95system 0:14.19elapsed 99%CPU without the patch, and
1.18user 9.37system 0:10.57elapsed 99%CPU with the patch.

This is not a serious benchmark, but can reflect the performance
difference.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
---
 fs/btrfs/ioctl.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 9948a12fd5e3..27c294e9d68d 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1502,8 +1502,10 @@ static noinline int copy_to_sk(struct btrfs_path *path,
 		item_len = btrfs_item_size(leaf, i);
 
 		btrfs_item_key_to_cpu(leaf, key, i);
-		if (!key_in_sk(key, sk))
-			continue;
+		if (!key_in_sk(key, sk)) {
+			ret = 1;
+			goto out;
+		}
 
 		if (sizeof(sh) + item_len > *buf_size) {
 			if (*num_found) {
-- 
2.50.0