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

[yebin]

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@xxxxxxxxxx>
> 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