Re: [PATCH -next v2 6/6] ext4: fix possible store wrong check interval value in disk when umount

From: Jan Kara
Date: Thu Oct 07 2021 - 09:12:13 EST


On Sat 11-09-21 17:00:59, Ye Bin wrote:
> Test follow steps:
> 1. mkfs.ext4 /dev/sda -O mmp
> 2. mount /dev/sda /mnt
> 3. wait for about 1 minute
> 4. umount mnt
> 5. debugfs /dev/sda
> 6. dump_mmp
> 7. fsck.ext4 /dev/sda
>
> I found 'check_interval' is range in [5, 10]. And sometime run fsck
> print "MMP interval is 10 seconds and total wait time is 42 seconds.
> Please wait...".
> kmmpd:
> ...
> if (diff < mmp_update_interval * HZ)
> schedule_timeout_interruptible(mmp_update_interval * HZ - diff);
> diff = jiffies - last_update_time;
> ...
> mmp_check_interval = max(min(EXT4_MMP_CHECK_MULT * diff / HZ,
> EXT4_MMP_MAX_CHECK_INTERVAL),
> EXT4_MMP_MIN_CHECK_INTERVAL);
> mmp->mmp_check_interval = cpu_to_le16(mmp_check_interval);
> ...
> We will call ext4_stop_mmpd to stop kmmpd kthread when umount, and
> schedule_timeout_interruptible will be interrupted, so 'diff' maybe
> little than mmp_update_interval. Then mmp_check_interval will range
> in [EXT4_MMP_MAX_CHECK_INTERVAL, EXT4_MMP_CHECK_MULT * diff / HZ].
> To solve this issue, if 'diff' little then mmp_update_interval * HZ
> just break loop, don't update check interval.
>
> Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
> ---
> fs/ext4/mmp.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
> index a0d47a906faa..f39e1fa0c6db 100644
> --- a/fs/ext4/mmp.c
> +++ b/fs/ext4/mmp.c
> @@ -205,6 +205,14 @@ static int kmmpd(void *data)
> schedule_timeout_interruptible(mmp_update_interval *
> HZ - diff);
> diff = jiffies - last_update_time;
> + /* If 'diff' little 'than mmp_update_interval * HZ', it
> + * means someone call ext4_stop_mmpd to stop kmmpd
> + * kthread. We don't need to update mmp_check_interval
> + * any more, as 'diff' is not exact value.
> + */
> + if (unlikely(diff < mmp_update_interval * HZ &&
> + kthread_should_stop()))
> + break;
> }

So in this case, mmp_check_interval would be EXT4_MMP_MIN_CHECK_INTERVAL. I
don't quite understand what the practical problem is - the fsck message?
That will happen anytime mmp_check_interval is >= 10 AFAICT and I don't
quite see how that is connected to this condition... Can you explain a bit
more please?

Honza

--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR