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

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Date:   Fri, 8 Oct 2021 09:50:44 +0800
From:   yebin <yebin10@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <tytso@....edu>, <adilger.kernel@...ger.ca>,
        <linux-ext4@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next v2 1/6] ext4: init seq with random value in kmmpd



On 2021/10/7 20:26, Jan Kara wrote:
> 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
Thank you for your advice，i  will  fix it.
>
>> ---
>>   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
>>