Re: [PATCH] Need update superblock on time when deciding to do reshape

[Shaohua Li <shli@kernel.org>]

On Mon, May 23, 2016 at 05:19:41PM -0400, Xiao Ni wrote:
> 
> 
> ----- Original Message -----
> > From: "Shaohua Li" <shli@kernel.org>
> > To: "Xiao Ni" <xni@redhat.com>
> > Cc: linux-raid@vger.kernel.org, "Jes Sorensen" <Jes.Sorensen@redhat.com>
> > Sent: Tuesday, May 24, 2016 3:02:04 AM
> > Subject: Re: [PATCH] Need update superblock on time when deciding to do reshape
> > 
> > On Sat, May 21, 2016 at 10:14:07PM -0400, Xiao Ni wrote:
> > > 
> > > 
> > > ----- Original Message -----
> > > > From: "Shaohua Li" <shli@kernel.org>
> > > > To: "Xiao Ni" <xni@redhat.com>
> > > > Cc: linux-raid@vger.kernel.org, "Jes Sorensen" <Jes.Sorensen@redhat.com>
> > > > Sent: Saturday, May 21, 2016 1:59:46 AM
> > > > Subject: Re: [PATCH] Need update superblock on time when deciding to do
> > > > reshape
> > > > 
> > > > On Tue, May 17, 2016 at 04:54:09PM +0800, Xiao Ni wrote:
> > > > > Hi all
> > > > > 
> > > > > If the disks are not enough to have spaces for relocating the
> > > > > data_offset,
> > > > > it needs to run start_reshape and then run mdadm --grow --continue by
> > > > > systemd. But mdadm --grow --continue fails because it checkes that
> > > > > info->reshape_active is 0.
> > > > > 
> > > > > The info->reshape_active is set to 1 when the superblock feature_map
> > > > > have the flag MD_FEATURE_RESHAPE_ACTIVE. Superblock feature_map is set
> > > > > MD_FEATURE_RESHAPE_ACTIVE as mddev->reshape_position != MaxSector.
> > > > > 
> > > > > Function start_reshape calls raid5_start_reshape which changes
> > > > > mddev->reshape_position to 0. Then in md_check_recovery it updates the
> > > > > superblock to underlying devices. But there is a chance that the
> > > > > superblock
> > > > > haven't written to underlying devices, the mdadm reads the superblock
> > > > > data.
> > > > > So mdadm --grow --continue fails.
> > > > > 
> > > > > The steps to reproduce this:
> > > > > mdadm -CR /dev/md0 -l5 -n3 /dev/loop[0-2] --bitmap=internal
> > > > > mdadm --wait /dev/md0
> > > > > mdadm /dev/md0 -a /dev/loop3
> > > > > mdadm --grow --raid-devices  4 /dev/md0
> > > > > The loop device size is 500MB
> > > > > 
> > > > > [root@storageqe-09 ~]# cat /proc/mdstat
> > > > > Personalities : [raid6] [raid5] [raid4]
> > > > > md0 : active raid5 loop3[4] loop2[3] loop1[1] loop0[0]
> > > > >       1021952 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/4]
> > > > >       [UUUU]
> > > > >       [>....................]  reshape =  0.0% (1/510976) finish=0.0min
> > > > >       speed=255488K/sec
> > > > >       bitmap: 1/1 pages [4KB], 65536KB chunk
> > > > 
> > > > what's the bad effect of the --continue failure? I think reshape will
> > > > still
> > > > continue. Doing a update super there is ok, but I'm wondering if it's the
> > > > good
> > > > way. Could mdadm wait for MD_FEATURE_RESHAPE_ACTIVE then let systemd run?
> > > > Because sounds like we are working around systemd bug, kernel itself will
> > > > write
> > > > superblock anyway soon, so we probably working around in userspace.
> > > 
> > > There is no bad effect if --continue failure. It just miss one chance to
> > > go on reshaping. Yes, as you said we can fix this in userspace. I tried
> > > to start mdadm-grow-continue@.service 30 seconds later as
> > > mdadm-last-resort@.service
> > > does. It can fix this too.
> > > 
> > > Sure it should be fixed this if mdadm try more times to check
> > > MD_FEATURE_RESHAPE_ACTIVE.
> > >  
> > > > 
> > > > > unused devices: <none>
> > > > > 
> > > > > So if we update the superblock on time, mdadm can read the right
> > > > > superblock
> > > > > data.
> > > > > 
> > > > > Signed-off-by <xni@redhat.com>
> > > > > 
> > > > > ---
> > > > >  drivers/md/md.c | 1 +
> > > > >  1 file changed, 1 insertion(+)
> > > > > 
> > > > > diff --git a/drivers/md/md.c b/drivers/md/md.c
> > > > > index 14d3b37..7919606 100644
> > > > > --- a/drivers/md/md.c
> > > > > +++ b/drivers/md/md.c
> > > > > @@ -4350,6 +4350,7 @@ action_store(struct mddev *mddev, const char
> > > > > *page,
> > > > > size_t len)
> > > > >  			else {
> > > > >  				clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> > > > >  				err = mddev->pers->start_reshape(mddev);
> > > > > +				md_update_sb(mddev, 1);
> > > > 
> > > > write super even err != 0?
> > > 
> > > Ah, sorry for this. It should update superblock only err is 0.
> > > 
> > > At first I want to fix this in userspace, but I ask myself why shouldn't
> > > update
> > > the superblock once start_reshape returns. There are some guys waiting for
> > > the
> > > update. The function action_store should update the superblock in time to
> > > tell
> > > the colleagues what happens now. Then I checked the places where
> > > md_update_sb
> > > is called. I found it is indeed called in some places where the md device
> > > changes. So I sent the patch here.
> > > 
> > > By the way, in size_store it update the superblock when err != 0. Is it
> > > right
> > > to check err there?
> > 
> > I think we should. probably nobody noticed it before.
> > 
> 
> So does it mean you applied the way fixing the problem in kernel space? If
> you apply the method I'll re-send the patch.

No, I didn't. I thought you agree with fixing it in userspace. For the err != 0
checking, I mean we probably should do it for size_store if somebody wants to
fix it.