Re: [PATCH v4 2/3] btrfs: Split remaining space to discard in chunks

[Qu Wenruo <wqu@suse.com>]

在 2024/9/16 19:46, Luca Stefani 写道:
> 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 | 24 +++++++++++++++++++-----
>   1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index a5966324607d..cbe66d0acff8 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -1239,7 +1239,7 @@ static int btrfs_issue_discard(struct block_device *bdev, u64 start, u64 len,
>   			       u64 *discarded_bytes)
>   {
>   	int j, ret = 0;
> -	u64 bytes_left, end;
> +	u64 bytes_left, bytes_to_discard, end;
>   	u64 aligned_start = ALIGN(start, 1 << SECTOR_SHIFT);
>   
>   	/* Adjust the range to be aligned to 512B sectors if necessary. */
> @@ -1300,13 +1300,27 @@ static int btrfs_issue_discard(struct block_device *bdev, u64 start, u64 len,
>   		bytes_left = end - start;
>   	}
>   
> -	if (bytes_left) {
> +	while (bytes_left) {
> +		if (bytes_left > BTRFS_MAX_DATA_CHUNK_SIZE)
> +			bytes_to_discard = BTRFS_MAX_DATA_CHUNK_SIZE;

That MAX_DATA_CHUNK_SIZE is only possible for RAID0/RAID10/RAID5/RAID6, 
by spanning the device extents across multiple devices.

For each device, the maximum size is limited to 1G (check 
init_alloc_chunk_ctl_policy_regular()).

So you can just limit it to 1G instead.
(If you want, you can also extract that into a macro as a cleanup).

Furthermore, you can use min() instead of a if ().

So you only need:

		bytes_to_discard = min(SZ_1G, bytes_left);

Otherwise this looks good enough to me.
If the 1G size is not good enough, we can later tune it to smaller values.

Personally speaking I think 1G would be enough.

Thanks,
Qu
> +		else
> +			bytes_to_discard = bytes_left;
> +
>   		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;
>   }
>