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

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

Ping?

Thanks, Anand

On 30/08/2021 21:11, Anand Jain wrote:
> 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;
>