Re: [PATCH] btrfs: reduce chunk_map lookups in btrfs_map_block

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2024/8/13 02:29, Johannes Thumshirn 写道:
> From: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>
> Currently we're calling btrfs_num_copies() before btrfs_get_chunk_map() in
> btrfs_map_block(). But btrfs_num_copies() itself does a chunk map lookup
> to be able to calculate the number of copies.
>
> So split out the code getting the number of copies from btrfs_num_copies()
> into a helper called btrfs_chunk_map_num_copies() and directly call it
> from btrfs_map_block() and btrfs_num_copies().
>
> This saves us one rbtree lookup per btrfs_map_block() invocation.
>
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Just one nitpick inlined below.

> ---
>   fs/btrfs/volumes.c | 50 +++++++++++++++++++++++++++-------------------
>   1 file changed, 29 insertions(+), 21 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index e07452207426..4863bdb4d6f4 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -5781,10 +5781,33 @@ void btrfs_mapping_tree_free(struct btrfs_fs_info *fs_info)
>   	write_unlock(&fs_info->mapping_tree_lock);
>   }
>
> +static int btrfs_chunk_map_num_copies(struct btrfs_chunk_map *map)
> +{
> +	enum btrfs_raid_types index = btrfs_bg_flags_to_raid_index(map->type);
> +
> +	/* Non-RAID56, use their ncopies from btrfs_raid_array. */
> +	if (!(map->type & BTRFS_BLOCK_GROUP_RAID56_MASK))
> +		return btrfs_raid_array[index].ncopies;
> +
> +	if (map->type & BTRFS_BLOCK_GROUP_RAID5)
> +		return 2;
> +
> +	/*
> +	 * There could be two corrupted data stripes, we need
> +	 * to loop retry in order to rebuild the correct data.
> +	 *
> +	 * Fail a stripe at a time on every retry except the
> +	 * stripe under reconstruction.
> +	 */
> +	if (map->type & BTRFS_BLOCK_GROUP_RAID6)
> +		return map->num_stripes;
> +
> +	return 1;

IIRC above if()s have checks all profiles.

Thus should we call ASSERT() instead? Or return map->num_stripes since
that's the only remaining possible case.

Thanks,
Qu
> +}
> +
>   int btrfs_num_copies(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
>   {
>   	struct btrfs_chunk_map *map;
> -	enum btrfs_raid_types index;
>   	int ret = 1;
>
>   	map = btrfs_get_chunk_map(fs_info, logical, len);
> @@ -5797,22 +5820,7 @@ int btrfs_num_copies(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
>   		 */
>   		return 1;
>
> -	index = btrfs_bg_flags_to_raid_index(map->type);
> -
> -	/* Non-RAID56, use their ncopies from btrfs_raid_array. */
> -	if (!(map->type & BTRFS_BLOCK_GROUP_RAID56_MASK))
> -		ret = btrfs_raid_array[index].ncopies;
> -	else if (map->type & BTRFS_BLOCK_GROUP_RAID5)
> -		ret = 2;
> -	else if (map->type & BTRFS_BLOCK_GROUP_RAID6)
> -		/*
> -		 * There could be two corrupted data stripes, we need
> -		 * to loop retry in order to rebuild the correct data.
> -		 *
> -		 * Fail a stripe at a time on every retry except the
> -		 * stripe under reconstruction.
> -		 */
> -		ret = map->num_stripes;
> +	ret = btrfs_chunk_map_num_copies(map);
>   	btrfs_free_chunk_map(map);
>   	return ret;
>   }
> @@ -6462,14 +6470,14 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
>   	io_geom.stripe_index = 0;
>   	io_geom.op = op;
>
> -	num_copies = btrfs_num_copies(fs_info, logical, fs_info->sectorsize);
> -	if (io_geom.mirror_num > num_copies)
> -		return -EINVAL;
> -
>   	map = btrfs_get_chunk_map(fs_info, logical, *length);
>   	if (IS_ERR(map))
>   		return PTR_ERR(map);
>
> +	num_copies = btrfs_chunk_map_num_copies(map);
> +	if (io_geom.mirror_num > num_copies)
> +		return -EINVAL;
> +
>   	map_offset = logical - map->start;
>   	io_geom.raid56_full_stripe_start = (u64)-1;
>   	max_len = btrfs_max_io_len(map, map_offset, &io_geom);