linux-kernel - Re: [PATCH v3 2/5] btrfs: split RAID stripes on deletion

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Date: Mon, 1 Jul 2024 10:07:09 -0400
From: Josef Bacik <josef@...icpanda.com>
To: Johannes Thumshirn <jth@...nel.org>
Cc: Chris Mason <clm@...com>, David Sterba <dsterba@...e.com>,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Johannes Thumshirn <johannes.thumshirn@....com>
Subject: Re: [PATCH v3 2/5] btrfs: split RAID stripes on deletion

On Mon, Jul 01, 2024 at 12:25:16PM +0200, Johannes Thumshirn wrote:
> From: Johannes Thumshirn <johannes.thumshirn@....com>
> 
> The current RAID stripe code assumes, that we will always remove a
> whole stripe entry.
> 
> But if we're only removing a part of a RAID stripe we're hitting the
> ASSERT()ion checking for this condition.
> 
> Instead of assuming the complete deletion of a RAID stripe, split the
> stripe if we need to.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....com>
> ---
>  fs/btrfs/ctree.c            |   1 +
>  fs/btrfs/raid-stripe-tree.c | 100 +++++++++++++++++++++++++++++++++-----------
>  2 files changed, 77 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index e33f9f5a228d..16f9cf6360a4 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -3863,6 +3863,7 @@ static noinline int setup_leaf_for_split(struct btrfs_trans_handle *trans,
>  	btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
>  
>  	BUG_ON(key.type != BTRFS_EXTENT_DATA_KEY &&
> +	       key.type != BTRFS_RAID_STRIPE_KEY &&
>  	       key.type != BTRFS_EXTENT_CSUM_KEY);
>  
>  	if (btrfs_leaf_free_space(leaf) >= ins_len)
> diff --git a/fs/btrfs/raid-stripe-tree.c b/fs/btrfs/raid-stripe-tree.c
> index 3020820dd6e2..64e36b46cbab 100644
> --- a/fs/btrfs/raid-stripe-tree.c
> +++ b/fs/btrfs/raid-stripe-tree.c
> @@ -33,42 +33,94 @@ int btrfs_delete_raid_extent(struct btrfs_trans_handle *trans, u64 start, u64 le
>  	if (!path)
>  		return -ENOMEM;
>  
> -	while (1) {
> -		key.objectid = start;
> -		key.type = BTRFS_RAID_STRIPE_KEY;
> -		key.offset = length;
> +again:
> +	key.objectid = start;
> +	key.type = BTRFS_RAID_STRIPE_KEY;
> +	key.offset = length;
>  
> -		ret = btrfs_search_slot(trans, stripe_root, &key, path, -1, 1);
> -		if (ret < 0)
> -			break;
> -		if (ret > 0) {
> -			ret = 0;
> -			if (path->slots[0] == 0)
> -				break;
> -			path->slots[0]--;
> -		}
> +	ret = btrfs_search_slot(trans, stripe_root, &key, path, -1, 1);
> +	if (ret < 0)
> +		goto out;
> +	if (ret > 0) {
> +		ret = 0;
> +		if (path->slots[0] == 0)
> +			goto out;
> +		path->slots[0]--;
> +	}
> +
> +	leaf = path->nodes[0];
> +	slot = path->slots[0];
> +	btrfs_item_key_to_cpu(leaf, &key, slot);
> +	found_start = key.objectid;
> +	found_end = found_start + key.offset;
> +
> +	/* That stripe ends before we start, we're done. */
> +	if (found_end <= start)
> +		goto out;
> +
> +	trace_btrfs_raid_extent_delete(fs_info, start, end,
> +				       found_start, found_end);
> +
> +	if (found_start < start) {
> +		u64 diff = start - found_start;
> +		struct btrfs_key new_key;
> +		int num_stripes;
> +		struct btrfs_stripe_extent *stripe_extent;
> +
> +		new_key.objectid = start;
> +		new_key.type = BTRFS_RAID_STRIPE_KEY;
> +		new_key.offset = length - diff;
> +
> +		ret = btrfs_duplicate_item(trans, stripe_root, path,
> +					   &new_key);
> +		if (ret)
> +			goto out;
>  
>  		leaf = path->nodes[0];
>  		slot = path->slots[0];
> -		btrfs_item_key_to_cpu(leaf, &key, slot);
> -		found_start = key.objectid;
> -		found_end = found_start + key.offset;
>  
> -		/* That stripe ends before we start, we're done. */
> -		if (found_end <= start)
> -			break;
> +		num_stripes =
> +			btrfs_num_raid_stripes(btrfs_item_size(leaf, slot));
> +		stripe_extent =
> +			btrfs_item_ptr(leaf, slot, struct btrfs_stripe_extent);
> +
> +		for (int i = 0; i < num_stripes; i++) {
> +			struct btrfs_raid_stride *raid_stride =
> +				&stripe_extent->strides[i];
> +			u64 physical =
> +				btrfs_raid_stride_physical(leaf, raid_stride);
> +
> +			btrfs_set_raid_stride_physical(leaf, raid_stride,
> +							     physical + diff);
> +		}
> +
> +		btrfs_mark_buffer_dirty(trans, leaf);
> +		btrfs_release_path(path);
> +		goto again;
> +	}
> +
> +	if (found_end > end) {
> +		u64 diff = found_end - end;
> +		struct btrfs_key new_key;
>  
> -		trace_btrfs_raid_extent_delete(fs_info, start, end,
> -					       found_start, found_end);
> +		new_key.objectid = found_start;
> +		new_key.type = BTRFS_RAID_STRIPE_KEY;
> +		new_key.offset = length - diff;
>  
> -		ASSERT(found_start >= start && found_end <= end);
> -		ret = btrfs_del_item(trans, stripe_root, path);
> +		ret = btrfs_duplicate_item(trans, stripe_root, path,
> +					   &new_key);

This seems incorrect to me.  If we have [0, 1MiB) and we're deleting [0,512KiB)
then the tree at this point is

[0, BTRFS_RAID_STRIPE_KEY, 512KiB]
[0, BTRFS_RAID_STRIPE_KEY, 1MiB]

which is valid as far as key ordering goes, but is a violation of the raid
stripe tree design correct?  And then you do goto again, and then you'll delete

[0, BTRFS_RAID_STRIPE_KEY, 512KiB]

but leave the old one in place, correct?  Thanks,

Josef