[PATCH] fs: record task name which froze superblock

[PATCH] fs: record task name which froze superblock

[Alexey Dobriyan]

Freezing and thawing are separate system calls, task which is supposed
to thaw filesystem/superblock can disappear due to crash or not thaw
due to a bug. Record at least task name (we can't take task_struct
reference) to make support engineer's life easier.

Hopefully 16 bytes per superblock isn't much.

P.S.: Cc'ing GFS2 people just in case they want to correct
my understanding of GFS2 having async freeze code.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/ioctl.c         |   22 ++++++++++++++++------
 fs/super.c         |    2 ++
 include/linux/fs.h |    2 ++
 3 files changed, 20 insertions(+), 6 deletions(-)

--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
 static int ioctl_fsfreeze(struct file *filp)
 {
 	struct super_block *sb = file_inode(filp)->i_sb;
+	int rv;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
@@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
 		return -EOPNOTSUPP;
 
 	/* Freeze */
-	if (sb->s_op->freeze_super)
-		return sb->s_op->freeze_super(sb);
-	return freeze_super(sb);
+	if (sb->s_op->freeze_super) {
+		rv = sb->s_op->freeze_super(sb);
+		if (rv == 0)
+			get_task_comm(sb->s_writers.freeze_comm, current);
+	} else
+		rv = freeze_super(sb);
+	return rv;
 }
 
 static int ioctl_fsthaw(struct file *filp)
 {
 	struct super_block *sb = file_inode(filp)->i_sb;
+	int rv;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/* Thaw */
-	if (sb->s_op->thaw_super)
-		return sb->s_op->thaw_super(sb);
-	return thaw_super(sb);
+	if (sb->s_op->thaw_super) {
+		rv = sb->s_op->thaw_super(sb);
+		if (rv == 0)
+			memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
+	} else
+		rv = thaw_super(sb);
+	return rv;
 }
 
 /*
--- a/fs/super.c
+++ b/fs/super.c
@@ -1355,6 +1355,7 @@ int freeze_super(struct super_block *sb)
 	 * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
 	 */
 	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+	get_task_comm(sb->s_writers.freeze_comm, current);
 	up_write(&sb->s_umount);
 	return 0;
 }
@@ -1391,6 +1392,7 @@ int thaw_super(struct super_block *sb)
 
 out:
 	sb->s_writers.frozen = SB_UNFROZEN;
+	memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
 	smp_wmb();
 	wake_up(&sb->s_writers.wait_unfrozen);
 	deactivate_locked_super(sb);
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1221,6 +1221,8 @@ struct sb_writers {
 	int			frozen;		/* Is sb frozen? */
 	wait_queue_head_t	wait_unfrozen;	/* queue for waiting for
 						   sb to be thawed */
+	/* who froze superblock */
+	char			freeze_comm[16];
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	lock_map[SB_FREEZE_LEVELS];
 #endif

Re: [PATCH] fs: record task name which froze superblock

[Jan Kara]

On Sat 14-02-15 21:55:24, Alexey Dobriyan wrote:
> Freezing and thawing are separate system calls, task which is supposed
> to thaw filesystem/superblock can disappear due to crash or not thaw
> due to a bug. Record at least task name (we can't take task_struct
> reference) to make support engineer's life easier.
> 
> Hopefully 16 bytes per superblock isn't much.
> 
> P.S.: Cc'ing GFS2 people just in case they want to correct
> my understanding of GFS2 having async freeze code.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
  Hum, and when do you show the task name? Or do you expect that customer
takes a crashdump and support just finds it in memory?

> ---
> 
>  fs/ioctl.c         |   22 ++++++++++++++++------
>  fs/super.c         |    2 ++
>  include/linux/fs.h |    2 ++
>  3 files changed, 20 insertions(+), 6 deletions(-)
> 
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
>  static int ioctl_fsfreeze(struct file *filp)
>  {
>  	struct super_block *sb = file_inode(filp)->i_sb;
> +	int rv;
>  
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
> @@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
>  		return -EOPNOTSUPP;
>  
>  	/* Freeze */
> -	if (sb->s_op->freeze_super)
> -		return sb->s_op->freeze_super(sb);
> -	return freeze_super(sb);
> +	if (sb->s_op->freeze_super) {
> +		rv = sb->s_op->freeze_super(sb);
> +		if (rv == 0)
> +			get_task_comm(sb->s_writers.freeze_comm, current);
> +	} else
> +		rv = freeze_super(sb);
> +	return rv;
  Why don't you just set the name in ioctl_fsfreeze() in both cases?
Also you seem to be missing freezing / thawing in freeze/thaw_bdev()
functions.

>  }
>  
>  static int ioctl_fsthaw(struct file *filp)
>  {
>  	struct super_block *sb = file_inode(filp)->i_sb;
> +	int rv;
>  
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
>  
>  	/* Thaw */
> -	if (sb->s_op->thaw_super)
> -		return sb->s_op->thaw_super(sb);
> -	return thaw_super(sb);
> +	if (sb->s_op->thaw_super) {
> +		rv = sb->s_op->thaw_super(sb);
> +		if (rv == 0)
> +			memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
> +	} else
> +		rv = thaw_super(sb);
> +	return rv;
>  }
>  
>  /*
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1355,6 +1355,7 @@ int freeze_super(struct super_block *sb)
>  	 * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
>  	 */
>  	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
> +	get_task_comm(sb->s_writers.freeze_comm, current);
>  	up_write(&sb->s_umount);
>  	return 0;
>  }
> @@ -1391,6 +1392,7 @@ int thaw_super(struct super_block *sb)
>  
>  out:
>  	sb->s_writers.frozen = SB_UNFROZEN;
> +	memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
>  	smp_wmb();
>  	wake_up(&sb->s_writers.wait_unfrozen);
>  	deactivate_locked_super(sb);
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1221,6 +1221,8 @@ struct sb_writers {
>  	int			frozen;		/* Is sb frozen? */
>  	wait_queue_head_t	wait_unfrozen;	/* queue for waiting for
>  						   sb to be thawed */
> +	/* who froze superblock */
> +	char			freeze_comm[16];
  Here should be TASK_COMM_LEN, shouldn't it?

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

Re: [PATCH] fs: record task name which froze superblock

[Alexey Dobriyan]

On Mon, Feb 16, 2015 at 10:38:52AM +0100, Jan Kara wrote:
> On Sat 14-02-15 21:55:24, Alexey Dobriyan wrote:
> > Freezing and thawing are separate system calls, task which is supposed
> > to thaw filesystem/superblock can disappear due to crash or not thaw
> > due to a bug. Record at least task name (we can't take task_struct
> > reference) to make support engineer's life easier.
> > 
> > Hopefully 16 bytes per superblock isn't much.
> > 
> > P.S.: Cc'ing GFS2 people just in case they want to correct
> > my understanding of GFS2 having async freeze code.
> > 
> > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
>   Hum, and when do you show the task name? Or do you expect that customer
> takes a crashdump and support just finds it in memory?

Yeah, having at least something in crashdump is fine.

> > --- a/fs/ioctl.c
> > +++ b/fs/ioctl.c
> > @@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
> >  static int ioctl_fsfreeze(struct file *filp)
> >  {
> >  	struct super_block *sb = file_inode(filp)->i_sb;
> > +	int rv;
> >  
> >  	if (!capable(CAP_SYS_ADMIN))
> >  		return -EPERM;
> > @@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
> >  		return -EOPNOTSUPP;
> >  
> >  	/* Freeze */
> > -	if (sb->s_op->freeze_super)
> > -		return sb->s_op->freeze_super(sb);
> > -	return freeze_super(sb);
> > +	if (sb->s_op->freeze_super) {
> > +		rv = sb->s_op->freeze_super(sb);
> > +		if (rv == 0)
> > +			get_task_comm(sb->s_writers.freeze_comm, current);
> > +	} else
> > +		rv = freeze_super(sb);
> > +	return rv;
>   Why don't you just set the name in ioctl_fsfreeze() in both cases?

There are users of freeze_super() in GFS2 unless I'm misreading code.

> Also you seem to be missing freezing / thawing in freeze/thaw_bdev()
> functions.

You are correct. Resending patch (blockdev freezing tested with XFS).

> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -1221,6 +1221,8 @@ struct sb_writers {
> >  	int			frozen;		/* Is sb frozen? */
> >  	wait_queue_head_t	wait_unfrozen;	/* queue for waiting for
> >  						   sb to be thawed */
> > +	/* who froze superblock */
> > +	char			freeze_comm[16];
>   Here should be TASK_COMM_LEN, shouldn't it?

It will pull sched.h, dunno if we care about headers anymore.

	Alexey

[PATCH v2] fs: record task name which froze superblock

[Alexey Dobriyan]

Freezing and thawing are separate system calls, task which is supposed
to thaw filesystem/superblock can disappear due to crash or not thaw
due to a bug. Record at least task name (we can't take task_struct
reference) to make support engineer's life easier.

Hopefully 16 bytes per superblock isn't much.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/block_dev.c     |   12 ++++++++----
 fs/ioctl.c         |   22 ++++++++++++++++------
 fs/super.c         |    2 ++
 include/linux/fs.h |    2 ++
 4 files changed, 28 insertions(+), 10 deletions(-)

--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -227,9 +227,11 @@ struct super_block *freeze_bdev(struct block_device *bdev)
 	sb = get_active_super(bdev);
 	if (!sb)
 		goto out;
-	if (sb->s_op->freeze_super)
+	if (sb->s_op->freeze_super) {
 		error = sb->s_op->freeze_super(sb);
-	else
+		if (error == 0)
+			get_task_comm(sb->s_writers.freeze_comm, current);
+	} else
 		error = freeze_super(sb);
 	if (error) {
 		deactivate_super(sb);
@@ -267,9 +269,11 @@ int thaw_bdev(struct block_device *bdev, struct super_block *sb)
 	if (!sb)
 		goto out;
 
-	if (sb->s_op->thaw_super)
+	if (sb->s_op->thaw_super) {
 		error = sb->s_op->thaw_super(sb);
-	else
+		if (error == 0)
+			memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
+	} else
 		error = thaw_super(sb);
 	if (error) {
 		bdev->bd_fsfreeze_count++;
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
 static int ioctl_fsfreeze(struct file *filp)
 {
 	struct super_block *sb = file_inode(filp)->i_sb;
+	int rv;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
@@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
 		return -EOPNOTSUPP;
 
 	/* Freeze */
-	if (sb->s_op->freeze_super)
-		return sb->s_op->freeze_super(sb);
-	return freeze_super(sb);
+	if (sb->s_op->freeze_super) {
+		rv = sb->s_op->freeze_super(sb);
+		if (rv == 0)
+			get_task_comm(sb->s_writers.freeze_comm, current);
+	} else
+		rv = freeze_super(sb);
+	return rv;
 }
 
 static int ioctl_fsthaw(struct file *filp)
 {
 	struct super_block *sb = file_inode(filp)->i_sb;
+	int rv;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/* Thaw */
-	if (sb->s_op->thaw_super)
-		return sb->s_op->thaw_super(sb);
-	return thaw_super(sb);
+	if (sb->s_op->thaw_super) {
+		rv = sb->s_op->thaw_super(sb);
+		if (rv == 0)
+			memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
+	} else
+		rv = thaw_super(sb);
+	return rv;
 }
 
 /*
--- a/fs/super.c
+++ b/fs/super.c
@@ -1355,6 +1355,7 @@ int freeze_super(struct super_block *sb)
 	 * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
 	 */
 	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+	get_task_comm(sb->s_writers.freeze_comm, current);
 	up_write(&sb->s_umount);
 	return 0;
 }
@@ -1391,6 +1392,7 @@ int thaw_super(struct super_block *sb)
 
 out:
 	sb->s_writers.frozen = SB_UNFROZEN;
+	memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
 	smp_wmb();
 	wake_up(&sb->s_writers.wait_unfrozen);
 	deactivate_locked_super(sb);
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1221,6 +1221,8 @@ struct sb_writers {
 	int			frozen;		/* Is sb frozen? */
 	wait_queue_head_t	wait_unfrozen;	/* queue for waiting for
 						   sb to be thawed */
+	/* who froze superblock and forgot to thaw it */
+	char			freeze_comm[16];
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	lock_map[SB_FREEZE_LEVELS];
 #endif

Re: [PATCH] fs: record task name which froze superblock

[Jan Kara]

On Wed 18-02-15 10:34:55, Alexey Dobriyan wrote:
> On Mon, Feb 16, 2015 at 10:38:52AM +0100, Jan Kara wrote:
> > On Sat 14-02-15 21:55:24, Alexey Dobriyan wrote:
> > > Freezing and thawing are separate system calls, task which is supposed
> > > to thaw filesystem/superblock can disappear due to crash or not thaw
> > > due to a bug. Record at least task name (we can't take task_struct
> > > reference) to make support engineer's life easier.
> > > 
> > > Hopefully 16 bytes per superblock isn't much.
> > > 
> > > P.S.: Cc'ing GFS2 people just in case they want to correct
> > > my understanding of GFS2 having async freeze code.
> > > 
> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> >   Hum, and when do you show the task name? Or do you expect that customer
> > takes a crashdump and support just finds it in memory?
> 
> Yeah, having at least something in crashdump is fine.
  OK, then comment about this at freeze_comm[] definition so that it's
clear it isn't just set-but-never-read field.
 
> > > --- a/fs/ioctl.c
> > > +++ b/fs/ioctl.c
> > > @@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
> > >  static int ioctl_fsfreeze(struct file *filp)
> > >  {
> > >  	struct super_block *sb = file_inode(filp)->i_sb;
> > > +	int rv;
> > >  
> > >  	if (!capable(CAP_SYS_ADMIN))
> > >  		return -EPERM;
> > > @@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
> > >  		return -EOPNOTSUPP;
> > >  
> > >  	/* Freeze */
> > > -	if (sb->s_op->freeze_super)
> > > -		return sb->s_op->freeze_super(sb);
> > > -	return freeze_super(sb);
> > > +	if (sb->s_op->freeze_super) {
> > > +		rv = sb->s_op->freeze_super(sb);
> > > +		if (rv == 0)
> > > +			get_task_comm(sb->s_writers.freeze_comm, current);
> > > +	} else
> > > +		rv = freeze_super(sb);
> > > +	return rv;
> >   Why don't you just set the name in ioctl_fsfreeze() in both cases?
> 
> There are users of freeze_super() in GFS2 unless I'm misreading code.
  Yes, there are. The call in fs/gfs2/glops.c is in a call path from
->freeze_super() handler for GFS2 so that one is handled in
ioctl_fsfreeze() anyway. The call in fs/gfs2/sys.c is a way to freeze
filesystem via sysfs (dunno why GFS2 has to invent its own thing and ioctl
isn't enough). Steven? So having the logic in ioctl_fsfreeze(),
freeze_bdev() and freeze_store() in gfs2 seems to be enough.

> > Also you seem to be missing freezing / thawing in freeze/thaw_bdev()
> > functions.
> 
> You are correct. Resending patch (blockdev freezing tested with XFS).
> 
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -1221,6 +1221,8 @@ struct sb_writers {
> > >  	int			frozen;		/* Is sb frozen? */
> > >  	wait_queue_head_t	wait_unfrozen;	/* queue for waiting for
> > >  						   sb to be thawed */
> > > +	/* who froze superblock */
> > > +	char			freeze_comm[16];
> >   Here should be TASK_COMM_LEN, shouldn't it?
> 
> It will pull sched.h, dunno if we care about headers anymore.
  That's not ideal but IMHO better than having the value hardcoded here.
That is pretty fragile - i.e. think what happens when someone decides to
increase TASK_COMM_LEN...

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

Re: [PATCH] fs: record task name which froze superblock

[Steven Whitehouse]

Hi,

On 18/02/15 09:13, Jan Kara wrote:
> On Wed 18-02-15 10:34:55, Alexey Dobriyan wrote:
>> On Mon, Feb 16, 2015 at 10:38:52AM +0100, Jan Kara wrote:
>>> On Sat 14-02-15 21:55:24, Alexey Dobriyan wrote:
>>>> Freezing and thawing are separate system calls, task which is supposed
>>>> to thaw filesystem/superblock can disappear due to crash or not thaw
>>>> due to a bug. Record at least task name (we can't take task_struct
>>>> reference) to make support engineer's life easier.
>>>>
>>>> Hopefully 16 bytes per superblock isn't much.
>>>>
>>>> P.S.: Cc'ing GFS2 people just in case they want to correct
>>>> my understanding of GFS2 having async freeze code.
>>>>
>>>> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
>>>    Hum, and when do you show the task name? Or do you expect that customer
>>> takes a crashdump and support just finds it in memory?
>> Yeah, having at least something in crashdump is fine.
>    OK, then comment about this at freeze_comm[] definition so that it's
> clear it isn't just set-but-never-read field.
>   
>>>> --- a/fs/ioctl.c
>>>> +++ b/fs/ioctl.c
>>>> @@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
>>>>   static int ioctl_fsfreeze(struct file *filp)
>>>>   {
>>>>   	struct super_block *sb = file_inode(filp)->i_sb;
>>>> +	int rv;
>>>>   
>>>>   	if (!capable(CAP_SYS_ADMIN))
>>>>   		return -EPERM;
>>>> @@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
>>>>   		return -EOPNOTSUPP;
>>>>   
>>>>   	/* Freeze */
>>>> -	if (sb->s_op->freeze_super)
>>>> -		return sb->s_op->freeze_super(sb);
>>>> -	return freeze_super(sb);
>>>> +	if (sb->s_op->freeze_super) {
>>>> +		rv = sb->s_op->freeze_super(sb);
>>>> +		if (rv == 0)
>>>> +			get_task_comm(sb->s_writers.freeze_comm, current);
>>>> +	} else
>>>> +		rv = freeze_super(sb);
>>>> +	return rv;
>>>    Why don't you just set the name in ioctl_fsfreeze() in both cases?
>> There are users of freeze_super() in GFS2 unless I'm misreading code.
>    Yes, there are. The call in fs/gfs2/glops.c is in a call path from
> ->freeze_super() handler for GFS2 so that one is handled in
> ioctl_fsfreeze() anyway. The call in fs/gfs2/sys.c is a way to freeze
> filesystem via sysfs (dunno why GFS2 has to invent its own thing and ioctl
> isn't enough). Steven? So having the logic in ioctl_fsfreeze(),
> freeze_bdev() and freeze_store() in gfs2 seems to be enough.
>
The sysfs freeze thing is historical and strongly deprecated - I hope 
that we may be able to remove it one day,

Steve.

Re: [PATCH] fs: record task name which froze superblock

[Alexey Dobriyan]

On Wed, Feb 18, 2015 at 12:13 PM, Jan Kara <jack@suse.cz> wrote:
> On Wed 18-02-15 10:34:55, Alexey Dobriyan wrote:
>> On Mon, Feb 16, 2015 at 10:38:52AM +0100, Jan Kara wrote:
>> > On Sat 14-02-15 21:55:24, Alexey Dobriyan wrote:
>> > > Freezing and thawing are separate system calls, task which is supposed
>> > > to thaw filesystem/superblock can disappear due to crash or not thaw
>> > > due to a bug. Record at least task name (we can't take task_struct
>> > > reference) to make support engineer's life easier.
>> > >
>> > > Hopefully 16 bytes per superblock isn't much.
>> > >
>> > > P.S.: Cc'ing GFS2 people just in case they want to correct
>> > > my understanding of GFS2 having async freeze code.
>> > >
>> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
>> >   Hum, and when do you show the task name? Or do you expect that customer
>> > takes a crashdump and support just finds it in memory?
>>
>> Yeah, having at least something in crashdump is fine.
>   OK, then comment about this at freeze_comm[] definition so that it's
> clear it isn't just set-but-never-read field.

OK.

>> > > --- a/fs/ioctl.c
>> > > +++ b/fs/ioctl.c
>> > > @@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
>> > >  static int ioctl_fsfreeze(struct file *filp)
>> > >  {
>> > >   struct super_block *sb = file_inode(filp)->i_sb;
>> > > + int rv;
>> > >
>> > >   if (!capable(CAP_SYS_ADMIN))
>> > >           return -EPERM;
>> > > @@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
>> > >           return -EOPNOTSUPP;
>> > >
>> > >   /* Freeze */
>> > > - if (sb->s_op->freeze_super)
>> > > -         return sb->s_op->freeze_super(sb);
>> > > - return freeze_super(sb);
>> > > + if (sb->s_op->freeze_super) {
>> > > +         rv = sb->s_op->freeze_super(sb);
>> > > +         if (rv == 0)
>> > > +                 get_task_comm(sb->s_writers.freeze_comm, current);
>> > > + } else
>> > > +         rv = freeze_super(sb);
>> > > + return rv;
>> >   Why don't you just set the name in ioctl_fsfreeze() in both cases?
>>
>> There are users of freeze_super() in GFS2 unless I'm misreading code.
>   Yes, there are. The call in fs/gfs2/glops.c is in a call path from
> ->freeze_super() handler for GFS2 so that one is handled in
> ioctl_fsfreeze() anyway. The call in fs/gfs2/sys.c is a way to freeze
> filesystem via sysfs (dunno why GFS2 has to invent its own thing and ioctl
> isn't enough). Steven? So having the logic in ioctl_fsfreeze(),
> freeze_bdev() and freeze_store() in gfs2 seems to be enough.

Jan, my logic is as follows.

Recording freezer task name is not filesystem/device specific and
thus should be done in generic code. So no changes in GFS2.

freeze_super() is generic counterpart to filesystem/device
specific ->freeze_super() hook, look how they are paired.
It should recore freezer task name, so any future user
will not forget to do the same.

So it's in ioctl_fsfreeze(), freeze_bdev() and freeze_super().

>> > > --- a/include/linux/fs.h
>> > > +++ b/include/linux/fs.h
>> > > @@ -1221,6 +1221,8 @@ struct sb_writers {
>> > >   int                     frozen;         /* Is sb frozen? */
>> > >   wait_queue_head_t       wait_unfrozen;  /* queue for waiting for
>> > >                                              sb to be thawed */
>> > > + /* who froze superblock */
>> > > + char                    freeze_comm[16];
>> >   Here should be TASK_COMM_LEN, shouldn't it?
>>
>> It will pull sched.h, dunno if we care about headers anymore.
>   That's not ideal but IMHO better than having the value hardcoded here.
> That is pretty fragile - i.e. think what happens when someone decides to
> increase TASK_COMM_LEN...

TASK_COMM_LEN is userspace ABI via at least prctl(PR_SET_NAME).
I can formally move it to include/uapi/linux/sched.h.
This allows to not drag sched.h into fs.h for one tiny define.

    Alexey

Re: [PATCH] fs: record task name which froze superblock

[Jan Kara]

On Fri 20-02-15 14:42:29, Alexey Dobriyan wrote:
> On Wed, Feb 18, 2015 at 12:13 PM, Jan Kara <jack@suse.cz> wrote:
> >> > > --- a/fs/ioctl.c
> >> > > +++ b/fs/ioctl.c
> >> > > @@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
> >> > >  static int ioctl_fsfreeze(struct file *filp)
> >> > >  {
> >> > >   struct super_block *sb = file_inode(filp)->i_sb;
> >> > > + int rv;
> >> > >
> >> > >   if (!capable(CAP_SYS_ADMIN))
> >> > >           return -EPERM;
> >> > > @@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
> >> > >           return -EOPNOTSUPP;
> >> > >
> >> > >   /* Freeze */
> >> > > - if (sb->s_op->freeze_super)
> >> > > -         return sb->s_op->freeze_super(sb);
> >> > > - return freeze_super(sb);
> >> > > + if (sb->s_op->freeze_super) {
> >> > > +         rv = sb->s_op->freeze_super(sb);
> >> > > +         if (rv == 0)
> >> > > +                 get_task_comm(sb->s_writers.freeze_comm, current);
> >> > > + } else
> >> > > +         rv = freeze_super(sb);
> >> > > + return rv;
> >> >   Why don't you just set the name in ioctl_fsfreeze() in both cases?
> >>
> >> There are users of freeze_super() in GFS2 unless I'm misreading code.
> >   Yes, there are. The call in fs/gfs2/glops.c is in a call path from
> > ->freeze_super() handler for GFS2 so that one is handled in
> > ioctl_fsfreeze() anyway. The call in fs/gfs2/sys.c is a way to freeze
> > filesystem via sysfs (dunno why GFS2 has to invent its own thing and ioctl
> > isn't enough). Steven? So having the logic in ioctl_fsfreeze(),
> > freeze_bdev() and freeze_store() in gfs2 seems to be enough.
> 
> Jan, my logic is as follows.
> 
> Recording freezer task name is not filesystem/device specific and
> thus should be done in generic code. So no changes in GFS2.
> 
> freeze_super() is generic counterpart to filesystem/device
> specific ->freeze_super() hook, look how they are paired.
> It should recore freezer task name, so any future user
> will not forget to do the same.
> 
> So it's in ioctl_fsfreeze(), freeze_bdev() and freeze_super().
  Well, but this stores the name in two different levels - once in the top
level (ioctl_fsfreeze(), freeze_bdev()) and once in a place called from
there (freeze_super()). That just seems wrong to me. You can just record
the frozen process in freeze_super() and mandate that if someone manages to
freeze the fs without calling freeze_super() from his ->freeze_super()
handler (currently there isn't such filesystem), he is also responsible for
setting freezer name... Hmm?

> >> > > --- a/include/linux/fs.h
> >> > > +++ b/include/linux/fs.h
> >> > > @@ -1221,6 +1221,8 @@ struct sb_writers {
> >> > >   int                     frozen;         /* Is sb frozen? */
> >> > >   wait_queue_head_t       wait_unfrozen;  /* queue for waiting for
> >> > >                                              sb to be thawed */
> >> > > + /* who froze superblock */
> >> > > + char                    freeze_comm[16];
> >> >   Here should be TASK_COMM_LEN, shouldn't it?
> >>
> >> It will pull sched.h, dunno if we care about headers anymore.
> >   That's not ideal but IMHO better than having the value hardcoded here.
> > That is pretty fragile - i.e. think what happens when someone decides to
> > increase TASK_COMM_LEN...
> 
> TASK_COMM_LEN is userspace ABI via at least prctl(PR_SET_NAME).
> I can formally move it to include/uapi/linux/sched.h.
> This allows to not drag sched.h into fs.h for one tiny define.
  OK, moving the definition would be preferable to me (and IMO also a
cleanup).

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

Re: [PATCH] fs: record task name which froze superblock

[Alexey Dobriyan]

On Fri, Feb 20, 2015 at 01:15:22PM +0100, Jan Kara wrote:
> On Fri 20-02-15 14:42:29, Alexey Dobriyan wrote:
> > On Wed, Feb 18, 2015 at 12:13 PM, Jan Kara <jack@suse.cz> wrote:
> > >> > > --- a/fs/ioctl.c
> > >> > > +++ b/fs/ioctl.c
> > >> > > @@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
> > >> > >  static int ioctl_fsfreeze(struct file *filp)
> > >> > >  {
> > >> > >   struct super_block *sb = file_inode(filp)->i_sb;
> > >> > > + int rv;
> > >> > >
> > >> > >   if (!capable(CAP_SYS_ADMIN))
> > >> > >           return -EPERM;
> > >> > > @@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
> > >> > >           return -EOPNOTSUPP;
> > >> > >
> > >> > >   /* Freeze */
> > >> > > - if (sb->s_op->freeze_super)
> > >> > > -         return sb->s_op->freeze_super(sb);
> > >> > > - return freeze_super(sb);
> > >> > > + if (sb->s_op->freeze_super) {
> > >> > > +         rv = sb->s_op->freeze_super(sb);
> > >> > > +         if (rv == 0)
> > >> > > +                 get_task_comm(sb->s_writers.freeze_comm, current);
> > >> > > + } else
> > >> > > +         rv = freeze_super(sb);
> > >> > > + return rv;
> > >> >   Why don't you just set the name in ioctl_fsfreeze() in both cases?
> > >>
> > >> There are users of freeze_super() in GFS2 unless I'm misreading code.
> > >   Yes, there are. The call in fs/gfs2/glops.c is in a call path from
> > > ->freeze_super() handler for GFS2 so that one is handled in
> > > ioctl_fsfreeze() anyway. The call in fs/gfs2/sys.c is a way to freeze
> > > filesystem via sysfs (dunno why GFS2 has to invent its own thing and ioctl
> > > isn't enough). Steven? So having the logic in ioctl_fsfreeze(),
> > > freeze_bdev() and freeze_store() in gfs2 seems to be enough.
> > 
> > Jan, my logic is as follows.
> > 
> > Recording freezer task name is not filesystem/device specific and
> > thus should be done in generic code. So no changes in GFS2.
> > 
> > freeze_super() is generic counterpart to filesystem/device
> > specific ->freeze_super() hook, look how they are paired.
> > It should recore freezer task name, so any future user
> > will not forget to do the same.
> > 
> > So it's in ioctl_fsfreeze(), freeze_bdev() and freeze_super().
>   Well, but this stores the name in two different levels - once in the top
> level (ioctl_fsfreeze(), freeze_bdev()) and once in a place called from
> there (freeze_super()). That just seems wrong to me. You can just record
> the frozen process in freeze_super() and mandate that if someone manages to
> freeze the fs without calling freeze_super() from his ->freeze_super()
> handler (currently there isn't such filesystem), he is also responsible for
> setting freezer name... Hmm?

Jan I understand you. But I find stranger that GFS will have to record
freezer name.

I'm sending v3, hopefully final.

> > >> > > --- a/include/linux/fs.h
> > >> > > +++ b/include/linux/fs.h
> > >> > > @@ -1221,6 +1221,8 @@ struct sb_writers {
> > >> > >   int                     frozen;         /* Is sb frozen? */
> > >> > >   wait_queue_head_t       wait_unfrozen;  /* queue for waiting for
> > >> > >                                              sb to be thawed */
> > >> > > + /* who froze superblock */
> > >> > > + char                    freeze_comm[16];
> > >> >   Here should be TASK_COMM_LEN, shouldn't it?
> > >>
> > >> It will pull sched.h, dunno if we care about headers anymore.
> > >   That's not ideal but IMHO better than having the value hardcoded here.
> > > That is pretty fragile - i.e. think what happens when someone decides to
> > > increase TASK_COMM_LEN...
> > 
> > TASK_COMM_LEN is userspace ABI via at least prctl(PR_SET_NAME).
> > I can formally move it to include/uapi/linux/sched.h.
> > This allows to not drag sched.h into fs.h for one tiny define.
>   OK, moving the definition would be preferable to me (and IMO also a
> cleanup).

[PATCH v3] fs: record task name which froze superblock

[Alexey Dobriyan]

Freezing and thawing are separate system calls, task which is supposed
to thaw filesystem/superblock can disappear due to crash or not thaw
due to a bug. At least record task name (we can't take task_struct
reference) to make support engineer's life easier.

Hopefully 16 bytes per superblock isn't much.

TASK_COMM_LEN definition (which is userspace ABI, see prctl(PR_SET_NAME)) is
moved to userspace exported header to not drag sched.h into every fs.h inclusion.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/block_dev.c             |   12 ++++++++----
 fs/ioctl.c                 |   22 ++++++++++++++++------
 fs/super.c                 |    2 ++
 include/linux/fs.h         |    6 ++++++
 include/linux/sched.h      |    3 ---
 include/uapi/linux/sched.h |    3 +++
 6 files changed, 35 insertions(+), 13 deletions(-)

--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -227,9 +227,11 @@ struct super_block *freeze_bdev(struct block_device *bdev)
 	sb = get_active_super(bdev);
 	if (!sb)
 		goto out;
-	if (sb->s_op->freeze_super)
+	if (sb->s_op->freeze_super) {
 		error = sb->s_op->freeze_super(sb);
-	else
+		if (error == 0)
+			get_task_comm(sb->s_writers.freeze_comm, current);
+	} else
 		error = freeze_super(sb);
 	if (error) {
 		deactivate_super(sb);
@@ -267,9 +269,11 @@ int thaw_bdev(struct block_device *bdev, struct super_block *sb)
 	if (!sb)
 		goto out;
 
-	if (sb->s_op->thaw_super)
+	if (sb->s_op->thaw_super) {
 		error = sb->s_op->thaw_super(sb);
-	else
+		if (error == 0)
+			memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
+	} else
 		error = thaw_super(sb);
 	if (error) {
 		bdev->bd_fsfreeze_count++;
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -518,6 +518,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
 static int ioctl_fsfreeze(struct file *filp)
 {
 	struct super_block *sb = file_inode(filp)->i_sb;
+	int rv;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
@@ -527,22 +528,31 @@ static int ioctl_fsfreeze(struct file *filp)
 		return -EOPNOTSUPP;
 
 	/* Freeze */
-	if (sb->s_op->freeze_super)
-		return sb->s_op->freeze_super(sb);
-	return freeze_super(sb);
+	if (sb->s_op->freeze_super) {
+		rv = sb->s_op->freeze_super(sb);
+		if (rv == 0)
+			get_task_comm(sb->s_writers.freeze_comm, current);
+	} else
+		rv = freeze_super(sb);
+	return rv;
 }
 
 static int ioctl_fsthaw(struct file *filp)
 {
 	struct super_block *sb = file_inode(filp)->i_sb;
+	int rv;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/* Thaw */
-	if (sb->s_op->thaw_super)
-		return sb->s_op->thaw_super(sb);
-	return thaw_super(sb);
+	if (sb->s_op->thaw_super) {
+		rv = sb->s_op->thaw_super(sb);
+		if (rv == 0)
+			memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
+	} else
+		rv = thaw_super(sb);
+	return rv;
 }
 
 /*
--- a/fs/super.c
+++ b/fs/super.c
@@ -1351,6 +1351,7 @@ int freeze_super(struct super_block *sb)
 	 * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
 	 */
 	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+	get_task_comm(sb->s_writers.freeze_comm, current);
 	up_write(&sb->s_umount);
 	return 0;
 }
@@ -1387,6 +1388,7 @@ int thaw_super(struct super_block *sb)
 
 out:
 	sb->s_writers.frozen = SB_UNFROZEN;
+	memset(sb->s_writers.freeze_comm, 0, TASK_COMM_LEN);
 	smp_wmb();
 	wake_up(&sb->s_writers.wait_unfrozen);
 	deactivate_locked_super(sb);
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -33,6 +33,7 @@
 
 #include <asm/byteorder.h>
 #include <uapi/linux/fs.h>
+#include <uapi/linux/sched.h>
 
 struct backing_dev_info;
 struct export_operations;
@@ -1217,6 +1218,11 @@ struct sb_writers {
 	int			frozen;		/* Is sb frozen? */
 	wait_queue_head_t	wait_unfrozen;	/* queue for waiting for
 						   sb to be thawed */
+	/*
+	 * who froze superblock
+	 * write-only field for traces in crashdump
+	 */
+	char			freeze_comm[TASK_COMM_LEN];
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	lock_map[SB_FREEZE_LEVELS];
 #endif
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -303,9 +303,6 @@ extern char ___assert_task_state[1 - 2*!!(
 
 #endif
 
-/* Task command name length */
-#define TASK_COMM_LEN 16
-
 #include <linux/spinlock.h>
 
 /*
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -49,4 +49,7 @@
  */
 #define SCHED_FLAG_RESET_ON_FORK	0x01
 
+/* Task command name length */
+#define TASK_COMM_LEN 16
+
 #endif /* _UAPI_LINUX_SCHED_H */

Re: [PATCH v3] fs: record task name which froze superblock

[Dave Chinner]

On Sat, Feb 28, 2015 at 05:25:57PM +0300, Alexey Dobriyan wrote:
> Freezing and thawing are separate system calls, task which is supposed
> to thaw filesystem/superblock can disappear due to crash or not thaw
> due to a bug. At least record task name (we can't take task_struct
> reference) to make support engineer's life easier.
> 
> Hopefully 16 bytes per superblock isn't much.
> 
> TASK_COMM_LEN definition (which is userspace ABI, see prctl(PR_SET_NAME)) is
> moved to userspace exported header to not drag sched.h into every fs.h inclusion.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>

Freeze/thaw can be nested at the block level. That means the
sb->s_writers.freeze_comm can point at the wrong process. i.e.

Task A			Task B
freeze_bdev
  freeze_super
    freeze_comm = A
			freeze_bdev
.....
thaw_bdev
 <device still frozen>
			<crash>

At this point, the block device will never be unthawed, but
the debug field is now pointing to the wrong task. i.e. The debug
helper has not recorded the process that is actually causing the
problem, and leads us all off on a wild goose chase down the wrong
path.

IMO, debug code is only useful if it's reliable.....

> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -303,9 +303,6 @@ extern char ___assert_task_state[1 - 2*!!(
>  
>  #endif
>  
> -/* Task command name length */
> -#define TASK_COMM_LEN 16
> -
>  #include <linux/spinlock.h>
>  
>  /*
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -49,4 +49,7 @@
>   */
>  #define SCHED_FLAG_RESET_ON_FORK	0x01
>  
> +/* Task command name length */
> +#define TASK_COMM_LEN 16
> +
>  #endif /* _UAPI_LINUX_SCHED_H */

That should be a separate patch, sent to the scheduler maintainers
for review. AFAICT, it isn't part of the user API - it's not defined
in the man page which just says "can be up to 16 bytes".

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

Re: [PATCH v3] fs: record task name which froze superblock

[Mateusz Guzik]

On Sun, Mar 01, 2015 at 08:31:26AM +1100, Dave Chinner wrote:
> On Sat, Feb 28, 2015 at 05:25:57PM +0300, Alexey Dobriyan wrote:
> > Freezing and thawing are separate system calls, task which is supposed
> > to thaw filesystem/superblock can disappear due to crash or not thaw
> > due to a bug. At least record task name (we can't take task_struct
> > reference) to make support engineer's life easier.
> > 
> > Hopefully 16 bytes per superblock isn't much.
> > 
> > TASK_COMM_LEN definition (which is userspace ABI, see prctl(PR_SET_NAME)) is
> > moved to userspace exported header to not drag sched.h into every fs.h inclusion.
> > 
> > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> 
> Freeze/thaw can be nested at the block level. That means the
> sb->s_writers.freeze_comm can point at the wrong process. i.e.
> 
> Task A			Task B
> freeze_bdev
>   freeze_super
>     freeze_comm = A
> 			freeze_bdev
> .....
> thaw_bdev
>  <device still frozen>
> 			<crash>
> 
> At this point, the block device will never be unthawed, but
> the debug field is now pointing to the wrong task. i.e. The debug
> helper has not recorded the process that is actually causing the
> problem, and leads us all off on a wild goose chase down the wrong
> path.
> 
> IMO, debug code is only useful if it's reliable.....
> 

It can be trivially modified to be very useful to support people.

Actually this patch clears saved task name on unfreeze, so in this
particular scenario we would end up with no data.

Freezer and unfreezer names don't even have to match, so there is not
much we can do here (e.g. recording all names in a linked list or
something is a non-starter because of this).

I propose the following:
- on freezing:
1. if 0->1 save the name
2. if 1->2 have a flag to note there is an additional freezer
- on unfreezing
1. if 1->0 clear the flag
2. DO NOT clear the name in any case

This way we keep the name for possible future reference and we know
whether something with this name was the sole freezer in this cycle.

As explained below, this one task name is already very useful and likely
covers majority of real life use cases.

While working in support we were getting a lot of vmcores where hung task
detector panicked the kernel because a lot of tasks were blocked
in UN state trying to write to frozen filesystems. I presume OP has
similar story.

Some back on forth commuication almost always revealed one process e.g.
freezing stuff and then blocking itself trying to access it. While we
could see it blocked, we had no presumptive evidence to pin freezing on
it. A matching name, while still not 100% conclusive, would be ok enough
to push the case forward and avoid a rountrip of systemap scripts
showing freezer process tree.

-- 
Mateusz Guzik

Re: [PATCH v3] fs: record task name which froze superblock

[Mateusz Guzik]

On Mon, Mar 02, 2015 at 05:38:29AM +0100, Mateusz Guzik wrote:
> On Sun, Mar 01, 2015 at 08:31:26AM +1100, Dave Chinner wrote:
> > On Sat, Feb 28, 2015 at 05:25:57PM +0300, Alexey Dobriyan wrote:
> > > Freezing and thawing are separate system calls, task which is supposed
> > > to thaw filesystem/superblock can disappear due to crash or not thaw
> > > due to a bug. At least record task name (we can't take task_struct
> > > reference) to make support engineer's life easier.
> > > 
> > > Hopefully 16 bytes per superblock isn't much.
> > > 
> > > TASK_COMM_LEN definition (which is userspace ABI, see prctl(PR_SET_NAME)) is
> > > moved to userspace exported header to not drag sched.h into every fs.h inclusion.
> > > 
> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> > 
> > Freeze/thaw can be nested at the block level. That means the
> > sb->s_writers.freeze_comm can point at the wrong process. i.e.
> > 
> > Task A			Task B
> > freeze_bdev
> >   freeze_super
> >     freeze_comm = A
> > 			freeze_bdev
> > .....
> > thaw_bdev
> >  <device still frozen>
> > 			<crash>
> > 
> > At this point, the block device will never be unthawed, but
> > the debug field is now pointing to the wrong task. i.e. The debug
> > helper has not recorded the process that is actually causing the
> > problem, and leads us all off on a wild goose chase down the wrong
> > path.
> > 
> > IMO, debug code is only useful if it's reliable.....
> > 
> 
> It can be trivially modified to be very useful to support people.
> 
> Actually this patch clears saved task name on unfreeze, so in this
> particular scenario we would end up with no data.
> 
> Freezer and unfreezer names don't even have to match, so there is not
> much we can do here (e.g. recording all names in a linked list or
> something is a non-starter because of this).
> 
> I propose the following:
> - on freezing:
> 1. if 0->1 save the name
> 2. if 1->2 have a flag to note there is an additional freezer
> - on unfreezing
> 1. if 1->0 clear the flag
> 2. DO NOT clear the name in any case
> 

Now that I sent this e-mail I realized we could actually keep a linked
list of freezer names. Unfreezing would delete all elements when going
1->0, but would not touch it otherwise.

This would cover a less likely use case though, so I would be fine
either way FWIW.

Just my $0,03.

> This way we keep the name for possible future reference and we know
> whether something with this name was the sole freezer in this cycle.
> 
> As explained below, this one task name is already very useful and likely
> covers majority of real life use cases.
> 
> While working in support we were getting a lot of vmcores where hung task
> detector panicked the kernel because a lot of tasks were blocked
> in UN state trying to write to frozen filesystems. I presume OP has
> similar story.
> 
> Some back on forth commuication almost always revealed one process e.g.
> freezing stuff and then blocking itself trying to access it. While we
> could see it blocked, we had no presumptive evidence to pin freezing on
> it. A matching name, while still not 100% conclusive, would be ok enough
> to push the case forward and avoid a rountrip of systemap scripts
> showing freezer process tree.
> 

-- 
Mateusz Guzik

Re: [PATCH v3] fs: record task name which froze superblock

[Dave Chinner]

On Mon, Mar 02, 2015 at 05:38:29AM +0100, Mateusz Guzik wrote:
> On Sun, Mar 01, 2015 at 08:31:26AM +1100, Dave Chinner wrote:
> > On Sat, Feb 28, 2015 at 05:25:57PM +0300, Alexey Dobriyan wrote:
> > > Freezing and thawing are separate system calls, task which is supposed
> > > to thaw filesystem/superblock can disappear due to crash or not thaw
> > > due to a bug. At least record task name (we can't take task_struct
> > > reference) to make support engineer's life easier.
> > > 
> > > Hopefully 16 bytes per superblock isn't much.
> > > 
> > > TASK_COMM_LEN definition (which is userspace ABI, see prctl(PR_SET_NAME)) is
> > > moved to userspace exported header to not drag sched.h into every fs.h inclusion.
> > > 
> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> > 
> > Freeze/thaw can be nested at the block level. That means the
> > sb->s_writers.freeze_comm can point at the wrong process. i.e.
> > 
> > Task A			Task B
> > freeze_bdev
> >   freeze_super
> >     freeze_comm = A
> > 			freeze_bdev
> > .....
> > thaw_bdev
> >  <device still frozen>
> > 			<crash>
> > 
> > At this point, the block device will never be unthawed, but
> > the debug field is now pointing to the wrong task. i.e. The debug
> > helper has not recorded the process that is actually causing the
> > problem, and leads us all off on a wild goose chase down the wrong
> > path.
> > 
> > IMO, debug code is only useful if it's reliable.....
> > 
> 
> It can be trivially modified to be very useful to support people.
> 
> Actually this patch clears saved task name on unfreeze, so in this
> particular scenario we would end up with no data.

It only clears it i thaw_super(), which is *not called* until the
last nested thaw_bdev() call is made.

When the system is hung what we actually need to know is who is
responsible for *thawing* the filesystem and then we can work out
why that hasn't run.  What this code tries to do is identify who
froze the filesystem and so indicate who *might* be responsible for
thawing it. If we mis-identify the agent who holds the freeze
status, then we fail to identify who needs to run the thaw and hence
we're still stuck not knowing WTF happened....

I understand why you want to record this - I'm not arguing that we
shouldn't do this. My point is that we should *make it reliable* and
not in any way ambiguous, otherwise we failed to solve the problem
it was intended for.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

Re: [PATCH v3] fs: record task name which froze superblock

[Alexey Dobriyan]

On Mon, Mar 2, 2015 at 7:38 AM, Mateusz Guzik <mguzik@redhat.com> wrote:

> As explained below, this one task name is already very useful and likely
> covers majority of real life use cases.
>
> While working in support we were getting a lot of vmcores where hung task
> detector panicked the kernel because a lot of tasks were blocked
> in UN state trying to write to frozen filesystems. I presume OP has
> similar story.

Yes, the intended "use" case is 1 freezer which hopefully covers
majority of bug reports.

> Some back on forth commuication almost always revealed one process e.g.
> freezing stuff and then blocking itself trying to access it. While we
> could see it blocked, we had no presumptive evidence to pin freezing on
> it. A matching name, while still not 100% conclusive, would be ok enough
> to push the case forward and avoid a rountrip of systemap scripts
> showing freezer process tree.

Re: [PATCH v3] fs: record task name which froze superblock

[Alexey Dobriyan]

On Mon, Mar 2, 2015 at 7:46 AM, Mateusz Guzik <mguzik@redhat.com> wrote:
> On Mon, Mar 02, 2015 at 05:38:29AM +0100, Mateusz Guzik wrote:
>> On Sun, Mar 01, 2015 at 08:31:26AM +1100, Dave Chinner wrote:
>> > On Sat, Feb 28, 2015 at 05:25:57PM +0300, Alexey Dobriyan wrote:
>> > > Freezing and thawing are separate system calls, task which is supposed
>> > > to thaw filesystem/superblock can disappear due to crash or not thaw
>> > > due to a bug. At least record task name (we can't take task_struct
>> > > reference) to make support engineer's life easier.
>> > >
>> > > Hopefully 16 bytes per superblock isn't much.
>> > >
>> > > TASK_COMM_LEN definition (which is userspace ABI, see prctl(PR_SET_NAME)) is
>> > > moved to userspace exported header to not drag sched.h into every fs.h inclusion.
>> > >
>> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
>> >
>> > Freeze/thaw can be nested at the block level. That means the
>> > sb->s_writers.freeze_comm can point at the wrong process. i.e.
>> >
>> > Task A                      Task B
>> > freeze_bdev
>> >   freeze_super
>> >     freeze_comm = A
>> >                     freeze_bdev
>> > .....
>> > thaw_bdev
>> >  <device still frozen>
>> >                     <crash>
>> >
>> > At this point, the block device will never be unthawed, but
>> > the debug field is now pointing to the wrong task. i.e. The debug
>> > helper has not recorded the process that is actually causing the
>> > problem, and leads us all off on a wild goose chase down the wrong
>> > path.
>> >
>> > IMO, debug code is only useful if it's reliable.....
>> >
>>
>> It can be trivially modified to be very useful to support people.
>>
>> Actually this patch clears saved task name on unfreeze, so in this
>> particular scenario we would end up with no data.
>>
>> Freezer and unfreezer names don't even have to match, so there is not
>> much we can do here (e.g. recording all names in a linked list or
>> something is a non-starter because of this).
>>
>> I propose the following:
>> - on freezing:
>> 1. if 0->1 save the name
>> 2. if 1->2 have a flag to note there is an additional freezer
>> - on unfreezing
>> 1. if 1->0 clear the flag
>> 2. DO NOT clear the name in any case
>>
>
> Now that I sent this e-mail I realized we could actually keep a linked
> list of freezer names. Unfreezing would delete all elements when going
> 1->0, but would not touch it otherwise.

If you do linked list two processes constantly keeping superblock frozen
will allocate all the memory:

F
  F
  T
  F
  T
 ...

After all valid objections and comments I think the way to proceed is
a) record freeze_comm, never clear it (maybe record thaw_comm)
b) record "i am not the only one" flag
c) introduce debug_freeze which will "record" everything in dmesg
for situations when crashdump is not used but serial console is.

    Alexey