Re: [PATCH 1/4] Btrfs: map the node block when looking for readahead targets

[Arne Jansen <sensille@gmx.net>]

On 11.05.2011 23:35, Josef Bacik wrote:
> If we have particularly full nodes, we could call btrfs_node_blockptr up to 32
> times, which is 32 pairs of kmap/kunmap, which _sucks_.  So go ahead and map the
> extent buffer while we look for readahead targets.  Thanks,
> 
> Signed-off-by: Josef Bacik <josef@redhat.com>
> ---
>  fs/btrfs/ctree.c |   23 +++++++++++++++++++++--
>  1 files changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index 84d7ca1..009bcf7 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -1229,6 +1229,7 @@ static void reada_for_search(struct btrfs_root *root,
>  	u64 search;
>  	u64 target;
>  	u64 nread = 0;
> +	u64 gen;
>  	int direction = path->reada;
>  	struct extent_buffer *eb;
>  	u32 nr;
> @@ -1256,6 +1257,15 @@ static void reada_for_search(struct btrfs_root *root,
>  	nritems = btrfs_header_nritems(node);
>  	nr = slot;
>  	while (1) {
> +		if (!node->map_token) {
> +			unsigned long offset = btrfs_node_key_ptr_offset(nr);
> +			map_private_extent_buffer(node, offset,
> +						  sizeof(struct btrfs_key_ptr),
> +						  &node->map_token,
> +						  &node->kaddr,
> +						  &node->map_start,
> +						  &node->map_len, KM_USER1);

You can't do that. It puts us in atomic context, and the following
readahead_tree_block will try a memory allocation with GFP_NOFS,
which leads to a BUG: sleeping function called from invalid context.
It didn't fall on our feet earlier because you also turned off
readahead, but scrub still uses it.

-Arne

> +		}
>  		if (direction < 0) {
>  			if (nr == 0)
>  				break;
> @@ -1273,14 +1283,23 @@ static void reada_for_search(struct btrfs_root *root,
>  		search = btrfs_node_blockptr(node, nr);
>  		if ((search <= target && target - search <= 65536) ||
>  		    (search > target && search - target <= 65536)) {
> -			readahead_tree_block(root, search, blocksize,
> -				     btrfs_node_ptr_generation(node, nr));
> +			gen = btrfs_node_ptr_generation(node, nr);
> +			if (node->map_token) {
> +				unmap_extent_buffer(node, node->map_token,
> +						    KM_USER1);
> +				node->map_token = NULL;
> +			}
> +			readahead_tree_block(root, search, blocksize, gen);
>  			nread += blocksize;
>  		}
>  		nscan++;
>  		if ((nread > 65536 || nscan > 32))
>  			break;
>  	}
> +	if (node->map_token) {
> +		unmap_extent_buffer(node, node->map_token, KM_USER1);
> +		node->map_token = NULL;
> +	}
>  }
>  
>  /*