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

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Message-ID: <20150218091323.GA4614@quack.suse.cz>
Date:	Wed, 18 Feb 2015 10:13:23 +0100
From:	Jan Kara <jack@...e.cz>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Jan Kara <jack@...e.cz>, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, swhiteho@...hat.com,
	cluster-devel@...hat.com
Subject: Re: [PATCH] fs: record task name which froze superblock

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@...il.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@...e.cz>
SUSE Labs, CR
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