Re: [PATCH] btrfs: reorder struct btrfs_key for better alignment

From: Qu Wenruo <wqu@suse.de>
To: David Sterba <DSterba@suse.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH] btrfs: reorder struct btrfs_key for better alignment
Date: Wed, 19 Jun 2019 09:37:39 +0800	[thread overview]
Message-ID: <6ba5daa9-735b-1cdd-fdd7-9c1a60277d46@suse.de> (raw)
In-Reply-To: <20190618141514.17322-1-dsterba@suse.com>

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On 2019/6/18 下午10:15, David Sterba wrote:
> We don't use the plain key for any on-disk operations so there's no
> requirement for the member order. As the offset is a u64 that should be
> on an 8byte aligned address, this can generate ineffective code on
> strict alignment architectures and can potentially hurt even on others
> (cross-cacheline access).
> 
> The resulting asm code on x86_64 only differes in the offset, no significant
> change in size of the object size.
> 
> The alignment of the structure is unchanged.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  include/uapi/linux/btrfs_tree.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
> index aff1356c2bb8..9ca7adcf3b7f 100644
> --- a/include/uapi/linux/btrfs_tree.h
> +++ b/include/uapi/linux/btrfs_tree.h
> @@ -342,10 +342,17 @@ struct btrfs_disk_key {
>  	__le64 offset;
>  } __attribute__ ((__packed__));
>  
> +/*
> + * NOTE: this structure does not match the on-disk format of key and must be
> + * converted with the right helpers. The btrfs_key is for in-memory use and the
> + * members are reordered for better alignment. It's still packed as it's never
> + * used in arrays and the extra alignment would consume stack space in
> + * functions.
> + */
>  struct btrfs_key {
>  	__u64 objectid;
> -	__u8 type;
>  	__u64 offset;
> +	__u8 type;
>  } __attribute__ ((__packed__));

And why not remove the packed attribute?

Thanks,
Qu
>  
>  struct btrfs_dev_item {
> 

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