Re: [PATCH v5 1/2] btrfs: Split remaining space to discard in chunks

[David Sterba <dsterba@suse.cz>]

On Mon, Sep 16, 2024 at 02:56:14PM +0200, Luca Stefani wrote:
> Per Qu Wenruo in case we have a very large disk, e.g. 8TiB device,
> mostly empty although we will do the split according to our super block
> locations, the last super block ends at 256G, we can submit a huge
> discard for the range [256G, 8T), causing a super large delay.
> 
> We now split the space left to discard based on BTRFS_MAX_DATA_CHUNK_SIZE
> in preparation of introduction of cancellation signals handling.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=219180
> Link: https://bugzilla.suse.com/show_bug.cgi?id=1229737
> Signed-off-by: Luca Stefani <luca.stefani.ge1@gmail.com>
> ---
>  fs/btrfs/extent-tree.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index a5966324607d..79b9243c9cd6 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -1300,13 +1300,24 @@ static int btrfs_issue_discard(struct block_device *bdev, u64 start, u64 len,
>  		bytes_left = end - start;
>  	}
>  
> -	if (bytes_left) {
> +	while (bytes_left) {
> +		u64 bytes_to_discard = min(SZ_1G, bytes_left);

Please define a separate constant for that and also mention it in the
changelog instead of BTRFS_MAX_DATA_CHUNK_SIZE.

> +
>  		ret = blkdev_issue_discard(bdev, start >> SECTOR_SHIFT,
> -					   bytes_left >> SECTOR_SHIFT,
> +					   bytes_to_discard >> SECTOR_SHIFT,
>  					   GFP_NOFS);
> -		if (!ret)
> -			*discarded_bytes += bytes_left;
> +
> +		if (ret) {
> +			if (ret != -EOPNOTSUPP)
> +				break;
> +			continue;
> +		}
> +
> +		start += bytes_to_discard;
> +		bytes_left -= bytes_to_discard;
> +		*discarded_bytes += bytes_to_discard;
>  	}
> +
>  	return ret;
>  }
>  
> -- 
> 2.46.0
>