Re: [PATCH v3 2/3] btrfs: Split remaining space to discard in chunks

[Luca Stefani <luca.stefani.ge1@gmail.com>]

On 03/09/24 09:16, 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 the block discard limit
> 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 | 24 +++++++++++++++++++-----
>   1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index a5966324607d..9c1ddf13659e 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -1301,12 +1301,26 @@ static int btrfs_issue_discard(struct block_device *bdev, u64 start, u64 len,
>   	}
>   
>   	if (bytes_left) {
> -		ret = blkdev_issue_discard(bdev, start >> SECTOR_SHIFT,
> -					   bytes_left >> SECTOR_SHIFT,
> -					   GFP_NOFS);
> -		if (!ret)
> -			*discarded_bytes += bytes_left;
I removed this by mistake, will be re-added.
> +		u64 bytes_to_discard;
> +		struct bio *bio = NULL;
> +		sector_t sector = start >> SECTOR_SHIFT;
> +		sector_t nr_sects = bytes_left >> SECTOR_SHIFT;
> +
> +		while ((bio = blk_alloc_discard_bio(bdev, &sector, &nr_sects,
> +				GFP_NOFS))) {
> +			ret = submit_bio_wait(bio);
> +			bio_put(bio);
> +
> +			if (!ret)
> +				bytes_to_discard = bio->bi_iter.bi_size;
> +			else if (ret != -EOPNOTSUPP)
> +				return ret;
I think I got the logic wrong, we probably want to `continue` in case 
ret is set, but it's not -EOPNOTSUPP, otherwise bytes_to_discard might 
be left uninitialized.
bio->bi_iter.bi_size can be used directly for all those cases, so I'll 
remove bytes_to_discard as a whole.
> +
> +			start += bytes_to_discard;
> +			bytes_left -= bytes_to_discard;
> +		}
>   	}
> +
>   	return ret;
>   }
>   

I'll fix those up for v4, but I'll wait for more comments before doing so.