linux-kernel - Re: [PATCH -next v2 1/6] ext4: init seq with random value in kmmpd

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Message-ID: <20211007122604.GF12712@quack2.suse.cz>
Date:   Thu, 7 Oct 2021 14:26:04 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ye Bin <yebin10@...wei.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        jack@...e.cz
Subject: Re: [PATCH -next v2 1/6] ext4: init seq with random value in kmmpd

On Sat 11-09-21 17:00:54, Ye Bin wrote:
> If two host has the same nodename, and seq start from 0, May cause the
> detection mechanism to fail.
> So init seq with random value to accelerate conflict detection.
> 
> Signed-off-by: Ye Bin <yebin10@...wei.com>

Thanks for the patch. I agree the code in kmmpd looks suspicious.  The
sequence number is initialized to a random number in
ext4_multi_mount_protect() which is called before kmmpd is started. I think
kmmpd() should initialize its 'seq' to a number fetched from the mmp
block, instead of 0 as is currently in the code. I don't think generating a
new random number as you do is really needed...

								Honza

> ---
>  fs/ext4/mmp.c | 32 ++++++++++++++++----------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
> index cebea4270817..12af6dc8457b 100644
> --- a/fs/ext4/mmp.c
> +++ b/fs/ext4/mmp.c
> @@ -122,6 +122,21 @@ void __dump_mmp_msg(struct super_block *sb, struct mmp_struct *mmp,
>  		       (int)sizeof(mmp->mmp_bdevname), mmp->mmp_bdevname);
>  }
>  
> +/*
> + * Get a random new sequence number but make sure it is not greater than
> + * EXT4_MMP_SEQ_MAX.
> + */
> +static unsigned int mmp_new_seq(void)
> +{
> +	u32 new_seq;
> +
> +	do {
> +		new_seq = prandom_u32();
> +	} while (new_seq > EXT4_MMP_SEQ_MAX);
> +
> +	return new_seq;
> +}
> +
>  /*
>   * kmmpd will update the MMP sequence every s_mmp_update_interval seconds
>   */
> @@ -132,7 +147,7 @@ static int kmmpd(void *data)
>  	struct buffer_head *bh = EXT4_SB(sb)->s_mmp_bh;
>  	struct mmp_struct *mmp;
>  	ext4_fsblk_t mmp_block;
> -	u32 seq = 0;
> +	u32 seq = mmp_new_seq();
>  	unsigned long failed_writes = 0;
>  	int mmp_update_interval = le16_to_cpu(es->s_mmp_update_interval);
>  	unsigned mmp_check_interval;
> @@ -258,21 +273,6 @@ void ext4_stop_mmpd(struct ext4_sb_info *sbi)
>  	}
>  }
>  
> -/*
> - * Get a random new sequence number but make sure it is not greater than
> - * EXT4_MMP_SEQ_MAX.
> - */
> -static unsigned int mmp_new_seq(void)
> -{
> -	u32 new_seq;
> -
> -	do {
> -		new_seq = prandom_u32();
> -	} while (new_seq > EXT4_MMP_SEQ_MAX);
> -
> -	return new_seq;
> -}
> -
>  /*
>   * Protect the filesystem from being mounted more than once.
>   */
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR