[RFC PATCH] ext4: add unmount filesystem message

[RFC PATCH] ext4: add unmount filesystem message

[Zhang Yi]

Now that we have kernel message at mount time, system administrator
could acquire the mount time, device and options easily. But we don't
have corresponding unmounting message at umount time, so we cannot know
if someone umount a filesystem easily. Some of the modern filesystems
(e.g. xfs) have the umounting kernel message, so add one for ext4
filesystem for convenience.

 EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
 EXT4-fs (sdb): unmounting filesystem.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
---
 fs/ext4/super.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 81749eaddf4c..bdecf62f4b55 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1199,6 +1199,9 @@ static void ext4_put_super(struct super_block *sb)
 	int aborted = 0;
 	int i, err;
 
+	if (___ratelimit(&ext4_mount_msg_ratelimit, "EXT4-fs unmount"))
+		ext4_msg(sb, KERN_INFO, "unmounting filesystem.");
+
 	ext4_unregister_li_request(sb);
 	ext4_quota_off_umount(sb);
 
-- 
2.31.1

Re: [RFC PATCH] ext4: add unmount filesystem message

[Gabriel Krisman Bertazi]

Zhang Yi <yi.zhang@huawei.com> writes:

> Now that we have kernel message at mount time, system administrator
> could acquire the mount time, device and options easily. But we don't
> have corresponding unmounting message at umount time, so we cannot know
> if someone umount a filesystem easily. Some of the modern filesystems
> (e.g. xfs) have the umounting kernel message, so add one for ext4
> filesystem for convenience.
>
>  EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
>  EXT4-fs (sdb): unmounting filesystem.

I don't think sysadmins should be relying on the kernel log for this,
since the information can easily be overwritten by new messages there.
Is there a reason why you can't just monitor /proc/self/mountinfo?

> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> ---
>  fs/ext4/super.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 81749eaddf4c..bdecf62f4b55 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1199,6 +1199,9 @@ static void ext4_put_super(struct super_block *sb)
>  	int aborted = 0;
>  	int i, err;
>  
> +	if (___ratelimit(&ext4_mount_msg_ratelimit, "EXT4-fs unmount"))
> +		ext4_msg(sb, KERN_INFO, "unmounting filesystem.");
> +
>  	ext4_unregister_li_request(sb);
>  	ext4_quota_off_umount(sb);

-- 
Gabriel Krisman Bertazi

Re: [RFC PATCH] ext4: add unmount filesystem message

[Theodore Ts'o]

On Tue, Apr 12, 2022 at 12:01:37PM -0400, Gabriel Krisman Bertazi wrote:
> Zhang Yi <yi.zhang@huawei.com> writes:
> 
> > Now that we have kernel message at mount time, system administrator

"Now that we have...." is a bit misleading, since (at least to an
English speaker) that this is something that was recently added, and
that's not the case.

> > could acquire the mount time, device and options easily. But we don't
> > have corresponding unmounting message at umount time, so we cannot know
> > if someone umount a filesystem easily. Some of the modern filesystems
> > (e.g. xfs) have the umounting kernel message, so add one for ext4
> > filesystem for convenience.
> >
> >  EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
> >  EXT4-fs (sdb): unmounting filesystem.
> 
> I don't think sysadmins should be relying on the kernel log for this,
> since the information can easily be overwritten by new messages there.
> Is there a reason why you can't just monitor /proc/self/mountinfo?

You're right that it can be dangerous for sysadmins to be relying on
the kernel log for mount and umount notifications --- but it depends
on what they think it means, and the potential pitfalls are there for
both the mount and unmount messages.  The problem of course, is that
bind mounts, and mount name spaces, so if the question is whether a
file system is available at a particular mount point, then using the
kernel log is definitely not going to be reliable.

But if the goal is to determine whether a particular device is safe to
run fsck or otherwise access directly, or for the purposes of
debugging the kernel and looking at the logs to understand when the
device is being accessed by the kernel and when the file system is
done with the device, I can see how it might be useful.

Cheers,

						- Ted

Re: [RFC PATCH] ext4: add unmount filesystem message

[Zhang Yi]

On 2022/4/13 9:35, Theodore Ts'o wrote:
> On Tue, Apr 12, 2022 at 12:01:37PM -0400, Gabriel Krisman Bertazi wrote:
>> Zhang Yi <yi.zhang@huawei.com> writes:
>>
>>> Now that we have kernel message at mount time, system administrator
> 
> "Now that we have...." is a bit misleading, since (at least to an
> English speaker) that this is something that was recently added, and
> that's not the case.
> 
>>> could acquire the mount time, device and options easily. But we don't
>>> have corresponding unmounting message at umount time, so we cannot know
>>> if someone umount a filesystem easily. Some of the modern filesystems
>>> (e.g. xfs) have the umounting kernel message, so add one for ext4
>>> filesystem for convenience.
>>>
>>>  EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
>>>  EXT4-fs (sdb): unmounting filesystem.
>>
>> I don't think sysadmins should be relying on the kernel log for this,
>> since the information can easily be overwritten by new messages there.
>> Is there a reason why you can't just monitor /proc/self/mountinfo?
> 
> You're right that it can be dangerous for sysadmins to be relying on
> the kernel log for mount and umount notifications --- but it depends
> on what they think it means, and the potential pitfalls are there for
> both the mount and unmount messages.  The problem of course, is that
> bind mounts, and mount name spaces, so if the question is whether a
> file system is available at a particular mount point, then using the
> kernel log is definitely not going to be reliable.
> 
> But if the goal is to determine whether a particular device is safe to
> run fsck or otherwise access directly, or for the purposes of
> debugging the kernel and looking at the logs to understand when the
> device is being accessed by the kernel and when the file system is
> done with the device, I can see how it might be useful.
> 

Yes, I understand that the kernel log is not reliable, and
/proc/self/mountinfo neither. Our goal is simple, As Ted said, just add a
method to help sysadmins to know whether a particular ext4 device is really
doing unmount procedure, it could be helpful for us to debug kernel and
locate kernel bug.

Thanks,
Yi.

Re: [RFC PATCH] ext4: add unmount filesystem message

[Darrick J. Wong]

On Wed, Apr 13, 2022 at 10:23:31AM +0800, Zhang Yi wrote:
> On 2022/4/13 9:35, Theodore Ts'o wrote:
> > On Tue, Apr 12, 2022 at 12:01:37PM -0400, Gabriel Krisman Bertazi wrote:
> >> Zhang Yi <yi.zhang@huawei.com> writes:
> >>
> >>> Now that we have kernel message at mount time, system administrator
> > 
> > "Now that we have...." is a bit misleading, since (at least to an
> > English speaker) that this is something that was recently added, and
> > that's not the case.
> > 
> >>> could acquire the mount time, device and options easily. But we don't
> >>> have corresponding unmounting message at umount time, so we cannot know
> >>> if someone umount a filesystem easily. Some of the modern filesystems
> >>> (e.g. xfs) have the umounting kernel message, so add one for ext4
> >>> filesystem for convenience.
> >>>
> >>>  EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
> >>>  EXT4-fs (sdb): unmounting filesystem.
> >>
> >> I don't think sysadmins should be relying on the kernel log for this,
> >> since the information can easily be overwritten by new messages there.
> >> Is there a reason why you can't just monitor /proc/self/mountinfo?
> > 
> > You're right that it can be dangerous for sysadmins to be relying on
> > the kernel log for mount and umount notifications --- but it depends
> > on what they think it means, and the potential pitfalls are there for
> > both the mount and unmount messages.  The problem of course, is that
> > bind mounts, and mount name spaces, so if the question is whether a
> > file system is available at a particular mount point, then using the
> > kernel log is definitely not going to be reliable.
> > 
> > But if the goal is to determine whether a particular device is safe to
> > run fsck or otherwise access directly, or for the purposes of
> > debugging the kernel and looking at the logs to understand when the
> > device is being accessed by the kernel and when the file system is
> > done with the device, I can see how it might be useful.
> > 
> 
> Yes, I understand that the kernel log is not reliable, and
> /proc/self/mountinfo neither. Our goal is simple, As Ted said, just add a
> method to help sysadmins to know whether a particular ext4 device is really
> doing unmount procedure, it could be helpful for us to debug kernel and
> locate kernel bug.

But if the mount/unmount messages are ratelimited, how will you know for
sure if the ratelimiting mechanism elides the message?

--D

> Thanks,
> Yi.
> 
> 
> 

Re: [RFC PATCH] ext4: add unmount filesystem message

[Zhang Yi]

On 2022/4/13 11:51, Darrick J. Wong wrote:
> On Wed, Apr 13, 2022 at 10:23:31AM +0800, Zhang Yi wrote:
>> On 2022/4/13 9:35, Theodore Ts'o wrote:
>>> On Tue, Apr 12, 2022 at 12:01:37PM -0400, Gabriel Krisman Bertazi wrote:
>>>> Zhang Yi <yi.zhang@huawei.com> writes:
>>>>
>>>>> Now that we have kernel message at mount time, system administrator
>>>
>>> "Now that we have...." is a bit misleading, since (at least to an
>>> English speaker) that this is something that was recently added, and
>>> that's not the case.
>>>
>>>>> could acquire the mount time, device and options easily. But we don't
>>>>> have corresponding unmounting message at umount time, so we cannot know
>>>>> if someone umount a filesystem easily. Some of the modern filesystems
>>>>> (e.g. xfs) have the umounting kernel message, so add one for ext4
>>>>> filesystem for convenience.
>>>>>
>>>>>  EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
>>>>>  EXT4-fs (sdb): unmounting filesystem.
>>>>
>>>> I don't think sysadmins should be relying on the kernel log for this,
>>>> since the information can easily be overwritten by new messages there.
>>>> Is there a reason why you can't just monitor /proc/self/mountinfo?
>>>
>>> You're right that it can be dangerous for sysadmins to be relying on
>>> the kernel log for mount and umount notifications --- but it depends
>>> on what they think it means, and the potential pitfalls are there for
>>> both the mount and unmount messages.  The problem of course, is that
>>> bind mounts, and mount name spaces, so if the question is whether a
>>> file system is available at a particular mount point, then using the
>>> kernel log is definitely not going to be reliable.
>>>
>>> But if the goal is to determine whether a particular device is safe to
>>> run fsck or otherwise access directly, or for the purposes of
>>> debugging the kernel and looking at the logs to understand when the
>>> device is being accessed by the kernel and when the file system is
>>> done with the device, I can see how it might be useful.
>>>
>>
>> Yes, I understand that the kernel log is not reliable, and
>> /proc/self/mountinfo neither. Our goal is simple, As Ted said, just add a
>> method to help sysadmins to know whether a particular ext4 device is really
>> doing unmount procedure, it could be helpful for us to debug kernel and
>> locate kernel bug.
> 
> But if the mount/unmount messages are ratelimited, how will you know for
> sure if the ratelimiting mechanism elides the message?
> 

This is to be expected that the messages are ratelimited, it's just a "try best"
way to let us acquire more information, it's best if it write something down and
not surprising if not. If the messages are ratelimited will get the "...suppressed"
message and could know what happened, we will combine other logs (e.g. systemd log)
to make things clear as far as possible.

Thanks,
Yi.

Re: [RFC PATCH] ext4: add unmount filesystem message

[Jan Kara]

On Wed 13-04-22 14:33:53, Zhang Yi wrote:
> On 2022/4/13 11:51, Darrick J. Wong wrote:
> > On Wed, Apr 13, 2022 at 10:23:31AM +0800, Zhang Yi wrote:
> >> On 2022/4/13 9:35, Theodore Ts'o wrote:
> >>> On Tue, Apr 12, 2022 at 12:01:37PM -0400, Gabriel Krisman Bertazi wrote:
> >>>> Zhang Yi <yi.zhang@huawei.com> writes:
> >>>>
> >>>>> Now that we have kernel message at mount time, system administrator
> >>>
> >>> "Now that we have...." is a bit misleading, since (at least to an
> >>> English speaker) that this is something that was recently added, and
> >>> that's not the case.
> >>>
> >>>>> could acquire the mount time, device and options easily. But we don't
> >>>>> have corresponding unmounting message at umount time, so we cannot know
> >>>>> if someone umount a filesystem easily. Some of the modern filesystems
> >>>>> (e.g. xfs) have the umounting kernel message, so add one for ext4
> >>>>> filesystem for convenience.
> >>>>>
> >>>>>  EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
> >>>>>  EXT4-fs (sdb): unmounting filesystem.
> >>>>
> >>>> I don't think sysadmins should be relying on the kernel log for this,
> >>>> since the information can easily be overwritten by new messages there.
> >>>> Is there a reason why you can't just monitor /proc/self/mountinfo?
> >>>
> >>> You're right that it can be dangerous for sysadmins to be relying on
> >>> the kernel log for mount and umount notifications --- but it depends
> >>> on what they think it means, and the potential pitfalls are there for
> >>> both the mount and unmount messages.  The problem of course, is that
> >>> bind mounts, and mount name spaces, so if the question is whether a
> >>> file system is available at a particular mount point, then using the
> >>> kernel log is definitely not going to be reliable.
> >>>
> >>> But if the goal is to determine whether a particular device is safe to
> >>> run fsck or otherwise access directly, or for the purposes of
> >>> debugging the kernel and looking at the logs to understand when the
> >>> device is being accessed by the kernel and when the file system is
> >>> done with the device, I can see how it might be useful.
> >>>
> >>
> >> Yes, I understand that the kernel log is not reliable, and
> >> /proc/self/mountinfo neither. Our goal is simple, As Ted said, just add a
> >> method to help sysadmins to know whether a particular ext4 device is really
> >> doing unmount procedure, it could be helpful for us to debug kernel and
> >> locate kernel bug.
> > 
> > But if the mount/unmount messages are ratelimited, how will you know for
> > sure if the ratelimiting mechanism elides the message?
> > 
> 
> This is to be expected that the messages are ratelimited, it's just a "try best"
> way to let us acquire more information, it's best if it write something down and
> not surprising if not. If the messages are ratelimited will get the "...suppressed"
> message and could know what happened, we will combine other logs (e.g. systemd log)
> to make things clear as far as possible.

Just to add my 2c, several times when I was debugging some issue and
staring into the kernel logs, I was trying to figure out whether some ext4
filesystem was still mounted or not and a message about unmounting a fs in
the kernel log would have been useful to me (e.g. when I was trying to
figure out whether a shared device with ext4 filesystem got really mounted
from two nodes at once or whether it was first unmounted on one of the
nodes). Sure, you can live without that and sure it isn't 100% reliable in
all the corner cases but it is convenient at times...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [RFC PATCH] ext4: add unmount filesystem message

[Jan Kara]

On Tue 12-04-22 22:53:20, Zhang Yi wrote:
> Now that we have kernel message at mount time, system administrator
> could acquire the mount time, device and options easily. But we don't
> have corresponding unmounting message at umount time, so we cannot know
> if someone umount a filesystem easily. Some of the modern filesystems
> (e.g. xfs) have the umounting kernel message, so add one for ext4
> filesystem for convenience.
> 
>  EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
>  EXT4-fs (sdb): unmounting filesystem.
> 
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>

Makes sense. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/ext4/super.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 81749eaddf4c..bdecf62f4b55 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1199,6 +1199,9 @@ static void ext4_put_super(struct super_block *sb)
>  	int aborted = 0;
>  	int i, err;
>  
> +	if (___ratelimit(&ext4_mount_msg_ratelimit, "EXT4-fs unmount"))
> +		ext4_msg(sb, KERN_INFO, "unmounting filesystem.");
> +
>  	ext4_unregister_li_request(sb);
>  	ext4_quota_off_umount(sb);
>  
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [RFC PATCH] ext4: add unmount filesystem message

[Theodore Ts'o]

On Tue, 12 Apr 2022 22:53:20 +0800, Zhang Yi wrote:
> Now that we have kernel message at mount time, system administrator
> could acquire the mount time, device and options easily. But we don't
> have corresponding unmounting message at umount time, so we cannot know
> if someone umount a filesystem easily. Some of the modern filesystems
> (e.g. xfs) have the umounting kernel message, so add one for ext4
> filesystem for convenience.
> 
> [...]

Applied, thanks!

[1/1] ext4: add unmount filesystem message
      commit: 4808cb5b98b436f1110d83c65541dd43beb45f63

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>