linux-ext4 - Re: [PATCH -v2] ext4: fix fencepost error in lazytime optimization

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Date:	Sun, 05 Jul 2015 12:04:59 +0200
From:	Jörg-Volker Peetz <jvpeetz@....de>
To:	linux-ext4@...r.kernel.org
Cc:	stable@...r.kernel.org
Subject: Re: [PATCH -v2] ext4: fix fencepost error in lazytime optimization

Hi,

Theodore Ts'o wrote on 07/05/2015 04:41:
> Commit 8f4d8558391: "ext4: fix lazytime optimization" was not a
> complete fix.  In the case where the inode number is a multiple of 16,
> and we could still end up updating an inode with dirty timestamps
> written to the wrong inode on disk.  Oops.
> 
> This can be easily reproduced by using generic/005 with a file system
> with metadata_csum and lazytime enabled.
> 
> Signed-off-by: Theodore Ts'o <tytso@....edu>
> Cc: stable@...r.kernel.org
> ---
>  fs/ext4/inode.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index e057c6f..4ad73d3 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4348,7 +4348,12 @@ static void ext4_update_other_inodes_time(struct super_block *sb,
>  	int inode_size = EXT4_INODE_SIZE(sb);
>  
>  	oi.orig_ino = orig_ino;
> -	ino = (orig_ino & ~(inodes_per_block - 1)) + 1;
> +	/*
> +	 * Calculate the first inode in the inode table block.  Inode
> +	 * numbers are one-based.  That is, the first inode in a block
> +	 * (assuming 4k blocks and 256 byte inodes) is (n*16 + 1).
> +	 */
> +	ino = ((orig_ino - 1) & ~(inodes_per_block - 1)) + 1;
>  	for (i = 0; i < inodes_per_block; i++, ino++, buf += inode_size) {
>  		if (ino == orig_ino)
>  			continue;
> 
thank you very much for the explanation. Now I think I understand:

/*
 * Calculate the first inode in the original inode's inode table
 * block. Inode numbers are one-based. That is, the first inode
 * in a block is one plus the next-lowest to (orig_ino - 1)
 * integral multiple of inodes_per_block. This can be calculated
 * efficiently by bit masking since inodes_per_block is a power
 * of 2 (assuming 4k blocks and 256 byte inodes).
*/

Regards,
Jörg.


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