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Date: Wed, 29 Nov 2023 22:20:43 +0100
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Reply-To: kreijack@inwind.it
Subject: Re: [PATCH RFC] btrfs: pick device with lowest devt for show_devname
Content-Language: en-US
To: Anand Jain <anand.jain@oracle.com>, linux-btrfs@vger.kernel.org
References: <85226cf68d7a72a034f0c0895b96b2557169755b.1698917826.git.anand.jain@oracle.com>
From: Goffredo Baroncelli <kreijack@libero.it>
In-Reply-To: <85226cf68d7a72a034f0c0895b96b2557169755b.1698917826.git.anand.jain@oracle.com>
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On 02/11/2023 12.10, Anand Jain wrote:
> In a non-single-device Btrfs filesystem, if Btrfs is already mounted and
> if you run the command 'mount -a,' it will fail and the command
> 'umount <device>' also fails. As below:
> 
> ----------------
> $ cat /etc/fstab | grep btrfs
> UUID=12345678-1234-1234-1234-123456789abc /btrfs btrfs defaults,nofail 0 0
> 
> $ mkfs.btrfs -qf --uuid 12345678-1234-1234-1234-123456789abc /dev/sda2 /dev/sda1
> $ mount --verbose -a
> /                        : ignored
> /btrfs                   : successfully mounted
> 
> $ ls -l /dev/disk/by-uuid | grep 12345678-1234-1234-1234-123456789abc
> lrwxrwxrwx 1 root root 10 Nov  2 17:43 12345678-1234-1234-1234-123456789abc -> ../../sda1
> 
> $ cat /proc/self/mounts | grep btrfs
> /dev/sda2 /btrfs btrfs rw,relatime,space_cache=v2,subvolid=5,subvol=/ 0 0
> 
> $ findmnt --df /btrfs
> SOURCE    FSTYPE SIZE  USED AVAIL USE% TARGET
> /dev/sda2 btrfs    2G  5.8M  1.8G   0% /btrfs
> 
> $ mount --verbose -a
> /                        : ignored
> mount: /btrfs: /dev/sda1 already mounted or mount point busy.
> $echo $?
> 32
> 
> $ umount /dev/sda1
> umount: /dev/sda1: not mounted.
> $ echo $?
> 32
> ----------------
> 
> I assume (RFC) this is because '/dev/disk/by-uuid,' '/proc/self/mounts,'
> and 'findmnt' do not all reference the same device, resulting in the
> 'mount -a' and 'umount' failures. However, an empirically found solution
> is to align them using a rule, such as the disk with the lowest 'devt,'
> for a multi-device Btrfs filesystem.

What about creating dedicated helper like mount.btrfs and umount.btrfs
that can manage all these kind of details ?

mount.btrfs may also help to have a better understand about problem
like a device missing.

> 
> I'm not yet sure (RFC) how to create a udev rule to point to the disk with
> the lowest 'devt,' as this kernel patch does, and I believe it is
> possible.
> 
> And this would ensure that '/proc/self/mounts,' 'findmnt,' and
> '/dev/disk/by-uuid' all reference the same device.
> 
> After applying this patch, the above test passes. Unfortunately,
> /dev/disk/by-uuid also points to the lowest 'devt' by chance, even though
> no rule has been set as of now. As shown below.
> 
> ----------------
> $ mkfs.btrfs -qf --uuid 12345678-1234-1234-1234-123456789abc /dev/sda2 /dev/sda1
> 
> $ mount --verbose -a
> /                        : ignored
> /btrfs                   : successfully mounted
> 
> $ ls -l /dev/disk/by-uuid | grep 12345678-1234-1234-1234-123456789abc
> lrwxrwxrwx 1 root root 10 Nov  2 17:53 12345678-1234-1234-1234-123456789abc -> ../../sda1
> 
> $ cat /proc/self/mounts | grep btrfs
> /dev/sda1 /btrfs btrfs rw,relatime,space_cache=v2,subvolid=5,subvol=/ 0 0
> 
> $ findmnt --df /btrfs
> SOURCE    FSTYPE SIZE  USED AVAIL USE% TARGET
> /dev/sda1 btrfs    2G  5.8M  1.8G   0% /btrfs
> 
> $ mount --verbose -a
> /                        : ignored
> /btrfs                   : already mounted
> $echo $?
> 0
> 
> $ umount /dev/sda1
> $echo $?
> 0
> ----------------
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>   fs/btrfs/super.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 66bdb6fd83bd..d768917cc5cc 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -2301,7 +2301,18 @@ 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_fs_devices *fs_devices = btrfs_sb(root->d_sb)->fs_devices;
> +	struct btrfs_device *device;
> +	struct btrfs_device *first_device = NULL;
> +
> +	list_for_each_entry(device, &fs_devices->devices, dev_list) {
> +		if (first_device == NULL) {
> +			first_device = device;
> +			continue;
> +		}
> +		if (first_device->devt > device->devt)
> +			first_device = device;
> +	}
>   
>   	/*
>   	 * There should be always a valid pointer in latest_dev, it may be stale
> @@ -2309,7 +2320,7 @@ static int btrfs_show_devname(struct seq_file *m, struct dentry *root)
>   	 * the end of RCU grace period.
>   	 */
>   	rcu_read_lock();
> -	seq_escape(m, btrfs_dev_name(fs_info->fs_devices->latest_dev), " \t\n\\");
> +	seq_escape(m, rcu_str_deref(first_device->name), " \t\n\\");
>   	rcu_read_unlock();
>   
>   	return 0;

-- 
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