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

[yebin]

On 2021/10/7 21:12, Jan Kara wrote:
> 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
I just think 'mmp_check_interval' is not reflect real check interval, 
and also sometime run fsck
print "MMP interval is 10 seconds and total wait time is 42 seconds. 
Please wait...", but
sometime not.