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

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Message-ID: <20150220121522.GC6293@quack.suse.cz>
Date:	Fri, 20 Feb 2015 13:15:22 +0100
From:	Jan Kara <jack@...e.cz>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	swhiteho@...hat.com, cluster-devel@...hat.com
Subject: Re: [PATCH] fs: record task name which froze superblock

On Fri 20-02-15 14:42:29, Alexey Dobriyan wrote:
> On Wed, Feb 18, 2015 at 12:13 PM, Jan Kara <jack@...e.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@...e.cz>
SUSE Labs, CR
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