[PATCH RFC V5 2/4] btrfs: use latest_dev in btrfs_show_devname

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

btrfs/238 reports warning as below.
[1]
------------[ cut here  ]------------
WARNING: CPU: 3 PID: 481 at fs/btrfs/super.c:2509 btrfs_show_devname+0x104/0x1e8 [btrfs]
CPU: 2 PID: 1 Comm: systemd Tainted: G        W  O 5.14.0-rc1-custom #72
Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
Call trace:
  btrfs_show_devname+0x108/0x1b4 [btrfs]
  show_mountinfo+0x234/0x2c4
  m_show+0x28/0x34
  seq_read_iter+0x12c/0x3c4
  vfs_read+0x29c/0x2c8
  ksys_read+0x80/0xec
  __arm64_sys_read+0x28/0x34
  invoke_syscall+0x50/0xf8
  do_el0_svc+0x88/0x138
  el0_svc+0x2c/0x8c
  el0t_64_sync_handler+0x84/0xe4
  el0t_64_sync+0x198/0x19c
---[ end trace 3efd7e5950b8af05  ]---

Reason:
While btrfs_prepare_sprout() moves the fs_devices::devices into
fs_devices::seed_list, the btrfs_show_devname() searched for the devices
and found none, leading to the warning as in [1] (above).

Fix:
latest_dev is updated according to the changes to the device list.
That means we could use the latest_dev->name to show the device name in
/proc/self/mounts. So this patch makes that change.

Reported-by: Su Yue <l@damenly.su>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
RFC because,
With this patch, /proc/self/mounts might not show the lowest devid
device as we did before. Instead we show the device that has the greatest
generation and, we used it to build the tree. IMO it is ok because
/proc/self/mounts should show a device the belongs to the fsid not,
necessarily the lowest devid device as devid is internal to btrfs.
IMO this won't affect the ABI?

 fs/btrfs/super.c | 24 +++---------------------
 1 file changed, 3 insertions(+), 21 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 64ecbdb50c1a..61682a143bf3 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2464,30 +2464,12 @@ static int btrfs_unfreeze(struct super_block *sb)
 static int btrfs_show_devname(struct seq_file *m, struct dentry *root)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(root->d_sb);
-	struct btrfs_device *dev, *first_dev = NULL;
 
-	/*
-	 * Lightweight locking of the devices. We should not need
-	 * device_list_mutex here as we only read the device data and the list
-	 * is protected by RCU.  Even if a device is deleted during the list
-	 * traversals, we'll get valid data, the freeing callback will wait at
-	 * least until the rcu_read_unlock.
-	 */
 	rcu_read_lock();
-	list_for_each_entry_rcu(dev, &fs_info->fs_devices->devices, dev_list) {
-		if (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state))
-			continue;
-		if (!dev->name)
-			continue;
-		if (!first_dev || dev->devid < first_dev->devid)
-			first_dev = dev;
-	}
-
-	if (first_dev)
-		seq_escape(m, rcu_str_deref(first_dev->name), " \t\n\\");
-	else
-		WARN_ON(1);
+	seq_escape(m, rcu_str_deref(fs_info->fs_devices->latest_dev->name),
+		   " \t\n\\");
 	rcu_read_unlock();
+
 	return 0;
 }
 
-- 
2.31.1