Re: [PATCH 4/5] btrfs: add fs flag to force device flushing

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

On 06/16/2017 12:49 AM, David Sterba wrote:
> We need a device capable of device barriers so we can test the flush
> code. To aid testing, add a per-filesystem status flag that affects the
> barriers regerdless of the device capabilities and obviously does not
> give the same guarantees.
> 
> It's off by default, sysfs tunable will follow.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>   fs/btrfs/ctree.h   | 1 +
>   fs/btrfs/disk-io.c | 8 +++++---
>   fs/btrfs/volumes.h | 1 +
>   3 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index f0f5f28784b6..dcf4404f7d61 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -716,6 +716,7 @@ struct btrfs_delayed_root;
>   #define BTRFS_FS_LOG1_ERR			12
>   #define BTRFS_FS_LOG2_ERR			13
>   #define BTRFS_FS_QUOTA_OVERRIDE			14
> +#define BTRFS_FS_FORCE_DEV_FLUSH		15
>   
>   /*
>    * Indicate that a whole-filesystem exclusive operation is running
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 59a732a13370..659a3b4645d2 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -3494,7 +3494,8 @@ static void write_dev_flush(struct btrfs_device *device)
>   	struct request_queue *q = bdev_get_queue(device->bdev);
>   	struct bio *bio = device->flush_bio;
>   
> -	if (!test_bit(QUEUE_FLAG_WC, &q->queue_flags))
> +	if (!test_bit(BTRFS_FS_FORCE_DEV_FLUSH, &device->fs_info->flags)
> +	    && !test_bit(QUEUE_FLAG_WC, &q->queue_flags))
>   		return;


  Now I understand what you meant. But the most common case in our test
  set up is a device with write cache. So BTRFS_FS_FORCE_DEV_FLUSH does
  not bring any additional force. IMO.

Thanks, Anand

>   	bio_reset(bio);
> @@ -3505,6 +3506,7 @@ static void write_dev_flush(struct btrfs_device *device)
>   	bio->bi_private = &device->flush_wait;
>   
>   	submit_bio(bio);
> +	device->flush_bio_sent = 1;
>   }
>   
>   /*
> @@ -3512,12 +3514,12 @@ static void write_dev_flush(struct btrfs_device *device)
>    */
>   static int wait_dev_flush(struct btrfs_device *device)
>   {
> -	struct request_queue *q = bdev_get_queue(device->bdev);
>   	struct bio *bio = device->flush_bio;
>   
> -	if (!test_bit(QUEUE_FLAG_WC, &q->queue_flags))
> +	if (!device->flush_bio_sent)
>   		return 0;
>   
> +	device->flush_bio_sent = 0;
>   	wait_for_completion_io(&device->flush_wait);
>   
>   	return bio->bi_error;
> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> index 35327efecdbb..6f45fd60d15a 100644
> --- a/fs/btrfs/volumes.h
> +++ b/fs/btrfs/volumes.h
> @@ -75,6 +75,7 @@ struct btrfs_device {
>   	int can_discard;
>   	int is_tgtdev_for_dev_replace;
>   	int last_flush_error;
> +	int flush_bio_sent;
>   
>   #ifdef __BTRFS_NEED_DEVICE_DATA_ORDERED
>   	seqcount_t data_seqcount;
>