linux-kernel - Re: [PATCH 1/2] erofs: add __packed annotation to union(_

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Message-ID: <20250409195222.4cadc368@pumpkin>
Date: Wed, 9 Apr 2025 19:52:22 +0100
From: David Laight <david.laight.linux@...il.com>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: linux-erofs@...ts.ozlabs.org, LKML <linux-kernel@...r.kernel.org>,
 kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/2] erofs: add __packed annotation to union(__le16..)

On Tue,  8 Apr 2025 19:44:47 +0800
Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:

> I'm unsure why they aren't 2 bytes in size only in arm-linux-gnueabi.

IIRC one of the arm ABI aligns structures on 32 bit boundaries.

> 
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/r/202504051202.DS7QIknJ-lkp@intel.com
> Fixes: 61ba89b57905 ("erofs: add 48-bit block addressing on-disk support")
> Fixes: efb2aef569b3 ("erofs: add encoded extent on-disk definition")
> Signed-off-by: Gao Xiang <hsiangkao@...ux.alibaba.com>
> ---
>  fs/erofs/erofs_fs.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
> index 61a5ee11f187..94bf636776b0 100644
> --- a/fs/erofs/erofs_fs.h
> +++ b/fs/erofs/erofs_fs.h
> @@ -56,7 +56,7 @@ struct erofs_super_block {
>  	union {
>  		__le16 rootnid_2b;	/* nid of root directory */
>  		__le16 blocks_hi;	/* (48BIT on) blocks count MSB */
> -	} rb;
> +	} __packed rb;
>  	__le64 inos;            /* total valid ino # (== f_files - f_favail) */
>  	__le64 epoch;		/* base seconds used for compact inodes */
>  	__le32 fixed_nsec;	/* fixed nanoseconds for compact inodes */
> @@ -148,7 +148,7 @@ union erofs_inode_i_nb {
>  	__le16 nlink;		/* if EROFS_I_NLINK_1_BIT is unset */
>  	__le16 blocks_hi;	/* total blocks count MSB */
>  	__le16 startblk_hi;	/* starting block number MSB */
> -};
> +} __packed;

That shouldn't be necessary and will kill performance on some systems.
The 'packed' on the member should be enough to reduce the size.

I'd add a compile assert (of some form) on the size of the structure.

	David

>  
>  /* 32-byte reduced form of an ondisk inode */
>  struct erofs_inode_compact {
> @@ -369,9 +369,9 @@ struct z_erofs_map_header {
>  			 * bit 7   : pack the whole file into packed inode
>  			 */
>  			__u8	h_clusterbits;
> -		};
> +		} __packed;
>  		__le16 h_extents_hi;	/* extent count MSB */
> -	};
> +	} __packed;

Ditto

>  };
>  
>  enum {