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remount-ro & umount & quota interaction

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mjt at tls

Feb 6, 2008, 9:55 AM

Post #1 of 8 (1547 views)
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remount-ro & umount & quota interaction

For a long time I'm bitten by a bad interaction
of mount -o remount,ro and quota operations.

The sequence is as follows:

mount /fs
quotaon -ug /fs
mount -o remount,ro /fs
umount /fs

At this point, umount never returns. /proc/$pid/wchan
shows vfs_quota_off:

Feb 6 20:53:25 linux kernel: umount D e5183eb8 0 8646 1
Feb 6 20:53:25 linux kernel: e5183ecc 00000086 00000002 e5183eb8 e5183eb0 00000000 c1db2540 c1db2684
Feb 6 20:53:25 linux kernel: c1db2684 c1c0dd00 00000000 cfd9f1c0 c0367080 c0367080 f5849000 f7f06880
Feb 6 20:53:25 linux kernel: f7e89d80 00000000 c0367080 b7c9795c 005f3997 00000000 000000ff 00000000
Feb 6 20:53:25 linux kernel: Call Trace:
Feb 6 20:53:25 linux kernel: [<c01a2a65>] vfs_quota_off+0x345/0x490
Feb 6 20:53:25 linux kernel: [<c013a3a0>] autoremove_wake_function+0x0/0x50
Feb 6 20:53:25 linux kernel: [<c0174bf6>] deactivate_super+0x46/0x80
Feb 6 20:53:25 linux kernel: [<c0188bba>] sys_umount+0x4a/0x240
Feb 6 20:53:25 linux kernel: [<c017637f>] sys_stat64+0xf/0x30
Feb 6 20:53:25 linux kernel: [<c0162069>] remove_vma+0x39/0x50
Feb 6 20:53:25 linux kernel: [<c0162b67>] do_munmap+0x197/0x1f0
Feb 6 20:53:25 linux kernel: [<c0188dc5>] sys_oldumount+0x15/0x20
Feb 6 20:53:25 linux kernel: [<c010417e>] sysenter_past_esp+0x5f/0x85

The filesystem is ext3. The issue is here for a long time,
at least since before 2.6.20, and is still present in 2.6.23
(I'll try 2.6.24 later today).

Can it be fixed please? :)

Thanks!

/mjt
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jack at suse

Feb 7, 2008, 2:28 AM

Post #2 of 8 (1482 views)
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Re: remount-ro & umount & quota interaction [In reply to]

> For a long time I'm bitten by a bad interaction
> of mount -o remount,ro and quota operations.
>
> The sequence is as follows:
>
> mount /fs
> quotaon -ug /fs
> mount -o remount,ro /fs
> umount /fs
>
> At this point, umount never returns. /proc/$pid/wchan
> shows vfs_quota_off:
>
> Feb 6 20:53:25 linux kernel: umount D e5183eb8 0 8646 1
> Feb 6 20:53:25 linux kernel: e5183ecc 00000086 00000002 e5183eb8 e5183eb0 00000000 c1db2540 c1db2684
> Feb 6 20:53:25 linux kernel: c1db2684 c1c0dd00 00000000 cfd9f1c0 c0367080 c0367080 f5849000 f7f06880
> Feb 6 20:53:25 linux kernel: f7e89d80 00000000 c0367080 b7c9795c 005f3997 00000000 000000ff 00000000
> Feb 6 20:53:25 linux kernel: Call Trace:
> Feb 6 20:53:25 linux kernel: [<c01a2a65>] vfs_quota_off+0x345/0x490
> Feb 6 20:53:25 linux kernel: [<c013a3a0>] autoremove_wake_function+0x0/0x50
> Feb 6 20:53:25 linux kernel: [<c0174bf6>] deactivate_super+0x46/0x80
> Feb 6 20:53:25 linux kernel: [<c0188bba>] sys_umount+0x4a/0x240
> Feb 6 20:53:25 linux kernel: [<c017637f>] sys_stat64+0xf/0x30
> Feb 6 20:53:25 linux kernel: [<c0162069>] remove_vma+0x39/0x50
> Feb 6 20:53:25 linux kernel: [<c0162b67>] do_munmap+0x197/0x1f0
> Feb 6 20:53:25 linux kernel: [<c0188dc5>] sys_oldumount+0x15/0x20
> Feb 6 20:53:25 linux kernel: [<c010417e>] sysenter_past_esp+0x5f/0x85
>
> The filesystem is ext3. The issue is here for a long time,
> at least since before 2.6.20, and is still present in 2.6.23
> (I'll try 2.6.24 later today).
>
> Can it be fixed please? :)
Of course, thanks for report :). The problem is we allow remounting
read only which we should refuse when quota is enabled. I'll fix that in
a minute.

Honza

--
Jan Kara <jack [at] suse>
SuSE CR Labs
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mjt at tls

Feb 7, 2008, 2:49 AM

Post #3 of 8 (1480 views)
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Re: remount-ro & umount & quota interaction [In reply to]

Jan Kara wrote:
[deadlock after remount-ro followed with umount when
quota is enabled]

> Of course, thanks for report :). The problem is we allow remounting
> read only which we should refuse when quota is enabled. I'll fix that in
> a minute.

Hmm. While that will prevent the lockup, maybe it's better to
perform an equivalent of quotaoff on mount-ro instead? Or even
do something more useful, like flush the quota stuff like the
rest of the filesystem is flushed to disk, so that on umount,
quota will not stay on the way...

I mean, why it locks in the first place? Quota subsystem trying
to write something into an read-only filesystem? If so, WHY it
is trying to do that on umount instead on a remount-ro?

Thanks!

/mjt
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jack at suse

Feb 7, 2008, 6:04 AM

Post #4 of 8 (1475 views)
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Re: remount-ro & umount & quota interaction [In reply to]

On Thu 07-02-08 13:49:52, Michael Tokarev wrote:
> Jan Kara wrote:
> [.deadlock after remount-ro followed with umount when
> quota is enabled]
>
> > Of course, thanks for report :). The problem is we allow remounting
> > read only which we should refuse when quota is enabled. I'll fix that in
> > a minute.
>
> Hmm. While that will prevent the lockup, maybe it's better to
> perform an equivalent of quotaoff on mount-ro instead? Or even
> do something more useful, like flush the quota stuff like the
> rest of the filesystem is flushed to disk, so that on umount,
> quota will not stay on the way...
We couldn't leave quota on when filesystem is remounted ro because we
need to modify quotafile when quota is being turned off. We could turn off
quotas when remounting read-only. As we turn them off during umount, it
probably makes sence to turn them off on remount-ro as well.

> I mean, why it locks in the first place? Quota subsystem trying
> to write something into an read-only filesystem? If so, WHY it
> is trying to do that on umount instead on a remount-ro?
Actually, I couldn't reproduce the hang on my testing machine so I don't
know exactly why it hangs. But my guess is that it's because we try to
write to the filesystem...

Honza
--
Jan Kara <jack [at] suse>
SUSE Labs, CR
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jengelh at computergmbh

Feb 7, 2008, 6:10 AM

Post #5 of 8 (1486 views)
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Re: remount-ro & umount & quota interaction [In reply to]

On Feb 7 2008 15:04, Jan Kara wrote:
>On Thu 07-02-08 13:49:52, Michael Tokarev wrote:
>> Jan Kara wrote:
>> [.deadlock after remount-ro followed with umount when
>> quota is enabled]
>>
>> Hmm. While that will prevent the lockup, maybe it's better to
>> perform an equivalent of quotaoff on mount-ro instead? [...]
>
> We couldn't leave quota on when filesystem is remounted ro because we
>need to modify quotafile when quota is being turned off. We could turn off
>quotas when remounting read-only. As we turn them off during umount, it
>probably makes sence to turn them off on remount-ro as well.

Objection. XFS handles quotas differently that does not involve
modifying a file on the fs, so quotas could stay on (even if it does
not make much sense) while the fs is ro.

(Hm, storing quota as files reminds me of the ugly xattr hack in
reiserfs3.)
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jack at suse

Feb 7, 2008, 6:32 AM

Post #6 of 8 (1467 views)
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Re: remount-ro & umount & quota interaction [In reply to]

On Thu 07-02-08 15:10:18, Jan Engelhardt wrote:
>
> On Feb 7 2008 15:04, Jan Kara wrote:
> >On Thu 07-02-08 13:49:52, Michael Tokarev wrote:
> >> Jan Kara wrote:
> >> [.deadlock after remount-ro followed with umount when
> >> quota is enabled]
> >>
> >> Hmm. While that will prevent the lockup, maybe it's better to
> >> perform an equivalent of quotaoff on mount-ro instead? [...]
> >
> > We couldn't leave quota on when filesystem is remounted ro because we
> >need to modify quotafile when quota is being turned off. We could turn off
> >quotas when remounting read-only. As we turn them off during umount, it
> >probably makes sence to turn them off on remount-ro as well.
>
> Objection. XFS handles quotas differently that does not involve
> modifying a file on the fs, so quotas could stay on (even if it does
> not make much sense) while the fs is ro.
Yes, but XFS doesn't give a damn about what we do in VFS with quotas ;)
So we are speaking here only about quotas implemented in VFS and these need
writing. BTW: When filesystem is remounted read-only, quota information
shouldn't change so it doesn't matter whether you turn it off or leave it
on. The only difference is that when you later remount rw, you have to turn
quotas on again.

> (Hm, storing quota as files reminds me of the ugly xattr hack in
> reiserfs3.)
Oh yes... there are some similarities ;). But quota was first! ;)

Honza
--
Jan Kara <jack [at] suse>
SUSE Labs, CR
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mjt at tls

Feb 7, 2008, 7:36 AM

Post #7 of 8 (1470 views)
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Re: remount-ro & umount & quota interaction [In reply to]

Jan Kara wrote:
[]
>> I mean, why it locks in the first place? Quota subsystem trying
>> to write something into an read-only filesystem? If so, WHY it
>> is trying to do that on umount instead on a remount-ro?

> Actually, I couldn't reproduce the hang on my testing machine so I don't
> know exactly why it hangs. But my guess is that it's because we try to
> write to the filesystem...

I can't reproduce it here easily as well. Yesterday I had a
locked-up console and had to hard-reboot the machine due to
this (it was far from first time when I've hit this issue),
but "on-demand reproducing" don't work (the uptime on that
host was about 100 days, and I had to do some repartition -
hence remount-ro to copy consistent data to other place -
maybe during that 100 day there was something... ;)

And I wasn't able to reproduce it on 2.6.24 so far, as well
(this one is only used on a test machine so far).

I'll keep trying ;)

Thanks for your support!

/mjt
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dgc at sgi

Feb 7, 2008, 4:32 PM

Post #8 of 8 (1474 views)
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Re: remount-ro & umount & quota interaction [In reply to]

On Thu, Feb 07, 2008 at 03:10:18PM +0100, Jan Engelhardt wrote:
>
> On Feb 7 2008 15:04, Jan Kara wrote:
> >On Thu 07-02-08 13:49:52, Michael Tokarev wrote:
> >> Jan Kara wrote:
> >> [.deadlock after remount-ro followed with umount when
> >> quota is enabled]
> >>
> >> Hmm. While that will prevent the lockup, maybe it's better to
> >> perform an equivalent of quotaoff on mount-ro instead? [...]
> >
> > We couldn't leave quota on when filesystem is remounted ro because we
> >need to modify quotafile when quota is being turned off. We could turn off
> >quotas when remounting read-only. As we turn them off during umount, it
> >probably makes sence to turn them off on remount-ro as well.
>
> Objection. XFS handles quotas differently that does not involve
> modifying a file on the fs, so quotas could stay on (even if it does
> not make much sense) while the fs is ro.

Wrong and right. XFS uses files for the backing store for quota
information, though it does handle quotas very differently to every
other Linux filesystem.

On remount-ro, we simply flush all the dirty dquots to their backing
store so the dquots can then be treated as ro just like every other object
in the filesystem. You don't need to turn off quotas to do this....

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group
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