Re: [PATCH RFC 4/6] btrfs: Check if the filesystem is has mixed type of devices

[Filipe Manana]

On Tue, Feb 9, 2021 at 9:32 PM Michal Rostecki <mrostecki@xxxxxxx> wrote:
>
> From: Michal Rostecki <mrostecki@xxxxxxxx>
>
> Add the btrfs_check_mixed() function which checks if the filesystem has
> the mixed type of devices (non-rotational and rotational). This
> information is going to be used in roundrobin raid1 read policy.
>
> Signed-off-by: Michal Rostecki <mrostecki@xxxxxxxx>
> ---
>  fs/btrfs/volumes.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
>  fs/btrfs/volumes.h |  7 +++++++
>  2 files changed, 49 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 1ac364a2f105..1ad30a595722 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -617,6 +617,35 @@ static int btrfs_free_stale_devices(const char *path,
>         return ret;
>  }
>
> +/*
> + * Checks if after adding the new device the filesystem is going to have mixed
> + * types of devices (non-rotational and rotational).
> + *
> + * @fs_devices:          list of devices
> + * @new_device_rotating: if the new device is rotational
> + *
> + * Returns true if there are mixed types of devices, otherwise returns false.
> + */
> +static bool btrfs_check_mixed(struct btrfs_fs_devices *fs_devices,
> +                             bool new_device_rotating)
> +{
> +       struct btrfs_device *device, *prev_device;
> +
> +       list_for_each_entry(device, &fs_devices->devices, dev_list) {
> +               if (prev_device == NULL &&

Hum, prev_device is not initialized when we enter the first iteration
of the loop.

> +                   device->rotating != new_device_rotating)
> +                       return true;
> +               if (prev_device != NULL &&
> +                   (device->rotating != prev_device->rotating ||

Here it's more dangerous, dereferencing an uninitialized pointer can
result in a crash.

With this fixed, it would be better to redo the benchmarks when using
mixed device types.

Thanks.

> +                    device->rotating != new_device_rotating))
> +                       return true;
> +
> +               prev_device = device;
> +       }
> +
> +       return false;
> +}
> +
>  /*
>   * This is only used on mount, and we are protected from competing things
>   * messing with our fs_devices by the uuid_mutex, thus we do not need the
> @@ -629,6 +658,7 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
>         struct request_queue *q;
>         struct block_device *bdev;
>         struct btrfs_super_block *disk_super;
> +       bool rotating;
>         u64 devid;
>         int ret;
>
> @@ -669,8 +699,12 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
>         }
>
>         q = bdev_get_queue(bdev);
> -       if (!blk_queue_nonrot(q))
> +       rotating = !blk_queue_nonrot(q);
> +       device->rotating = rotating;
> +       if (rotating)
>                 fs_devices->rotating = true;
> +       if (!fs_devices->mixed)
> +               fs_devices->mixed = btrfs_check_mixed(fs_devices, rotating);
>
>         device->bdev = bdev;
>         clear_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state);
> @@ -2418,6 +2452,7 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
>         fs_devices->open_devices = 0;
>         fs_devices->missing_devices = 0;
>         fs_devices->rotating = false;
> +       fs_devices->mixed = false;
>         list_add(&seed_devices->seed_list, &fs_devices->seed_list);
>
>         generate_random_uuid(fs_devices->fsid);
> @@ -2522,6 +2557,7 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
>         int seeding_dev = 0;
>         int ret = 0;
>         bool locked = false;
> +       bool rotating;
>
>         if (sb_rdonly(sb) && !fs_devices->seeding)
>                 return -EROFS;
> @@ -2621,8 +2657,12 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
>
>         atomic64_add(device->total_bytes, &fs_info->free_chunk_space);
>
> -       if (!blk_queue_nonrot(q))
> +       rotating = !blk_queue_nonrot(q);
> +       device->rotating = rotating;
> +       if (rotating)
>                 fs_devices->rotating = true;
> +       if (!fs_devices->mixed)
> +               fs_devices->mixed = btrfs_check_mixed(fs_devices, rotating);
>
>         orig_super_total_bytes = btrfs_super_total_bytes(fs_info->super_copy);
>         btrfs_set_super_total_bytes(fs_info->super_copy,
> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> index 6e544317a377..594f1207281c 100644
> --- a/fs/btrfs/volumes.h
> +++ b/fs/btrfs/volumes.h
> @@ -147,6 +147,9 @@ struct btrfs_device {
>         /* I/O stats for raid1 mirror selection */
>         struct percpu_counter inflight;
>         atomic_t last_offset;
> +
> +       /* If the device is rotational */
> +       bool rotating;
>  };
>
>  /*
> @@ -274,6 +277,10 @@ struct btrfs_fs_devices {
>          * nonrot flag set
>          */
>         bool rotating;
> +       /* Set when we find or add both nonrot and rot disks in the
> +        * filesystem
> +        */
> +       bool mixed;
>
>         struct btrfs_fs_info *fs_info;
>         /* sysfs kobjects */
> --
> 2.30.0
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”