[PATCH] btrfs: open_ctree: call btrfs_check_rw_degradable only if there is a missing device

[Anand Jain <anand.jain@oracle.com>]

In open_ctree() in btrfs_check_rw_degradable() [1], we check each block
group individually if at least the minimum number of devices is available
for that profile. However, if all the devices are available, then we don't
have to check degradable.

 [1]
 open_ctree()
 ::
 3559 if (!sb_rdonly(sb) && !btrfs_check_rw_degradable(fs_info, NULL)) {

Also before calling btrfs_check_rw_degradable() in open_ctee() at the
line number shown below [2] we call btrfs_read_chunk_tree() and down to
add_missing_dev() to record number of missing devices.

 [2]
 open_ctree()
 ::
 3454         ret = btrfs_read_chunk_tree(fs_info);

 btrfs_read_chunk_tree()
  read_one_chunk() / read_one_dev()
   add_missing_dev()

So, check if there is any missing device before btrfs_check_rw_degradable()
in open_ctree().

With this, in an example, the mount command could save ~16ms.[3] in the
most common case, that is no missing device.

[3]
 1) * 16934.96 us | btrfs_check_rw_degradable [btrfs]();

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/disk-io.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 4f38a657a30e..0ef3a8660ebd 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3556,7 +3556,8 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		goto fail_sysfs;
 	}
 
-	if (!sb_rdonly(sb) && !btrfs_check_rw_degradable(fs_info, NULL)) {
+	if (!sb_rdonly(sb) && fs_info->fs_devices->missing_devices &&
+	    !btrfs_check_rw_degradable(fs_info, NULL)) {
 		btrfs_warn(fs_info,
 		"writable mount is not allowed due to too many missing devices");
 		goto fail_sysfs;
-- 
2.31.1