Re: [RFC PATCH v2 2/2] overlayfs: propagate errors from upper to overlay sb in sync_fs

[Jeff Layton <jlayton@redhat.com>]

On Tue, 2020-12-15 at 11:30 -0500, Vivek Goyal wrote:
> On Mon, Dec 14, 2020 at 05:14:21PM -0500, Jeff Layton wrote:
> > Peek at the upper layer's errseq_t at mount time for volatile mounts,
> > and record it in the per-sb info. In sync_fs, check for an error since
> > the recorded point and set it in the overlayfs superblock if there was
> > one.
> > 
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> >  fs/overlayfs/ovl_entry.h |  1 +
> >  fs/overlayfs/super.c     | 19 ++++++++++++++-----
> >  2 files changed, 15 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> > index 1b5a2094df8e..f4285da50525 100644
> > --- a/fs/overlayfs/ovl_entry.h
> > +++ b/fs/overlayfs/ovl_entry.h
> > @@ -79,6 +79,7 @@ struct ovl_fs {
> >  	atomic_long_t last_ino;
> >  	/* Whiteout dentry cache */
> >  	struct dentry *whiteout;
> > +	errseq_t errseq;
> >  };
> >  
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >  static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs)
> > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> > index 290983bcfbb3..3f0cb91915ff 100644
> > --- a/fs/overlayfs/super.c
> > +++ b/fs/overlayfs/super.c
> > @@ -264,8 +264,16 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
> >  	if (!ovl_upper_mnt(ofs))
> >  		return 0;
> >  
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > -	if (!ovl_should_sync(ofs))
> > -		return 0;
> > +	upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
> > +
> > +	if (!ovl_should_sync(ofs)) {
> > +		/* Propagate errors from upper to overlayfs */
> > +		ret = errseq_check(&upper_sb->s_wb_err, ofs->errseq);
> > +		if (ret)
> > +			errseq_set(&sb->s_wb_err, ret);
> > +		return ret;
> > +	}
> > +
> 
> I have few concerns here. I think ovl_sync_fs() should not be different
> for volatile mounts and non-volatile mounts. IOW, if an overlayfs
> user calls syncfs(fd), then only difference with non-volatile mount
> is that we will not call sync_filesystem() on underlying filesystem. But
> if there is an existing writeback error then that should be reported
> to syncfs(fd) caller both in case of volatile and non-volatile mounts.
> 
> Additional requirement in case of non-volatile mount seems to be that
> as soon as we detect first error, we probably should mark whole file
> system bad and start returning error for overlay operations so that
> upper layer can be thrown away and process restarted.
> 

That was the reason the patch did the errseq_set on every sync_fs
invocation for a volatile mount. That should ensure that syncfs always
returns an error. Still, there probably are cleaner ways to do this...

> And final non-volatile mount requirement seems to that we want to detect
> writeback errors in non syncfs() paths, for ex. mount(). That's what
> Sargun is trying to do. Keep a snapshot of upper_sb errseq on disk
> and upon remount of volatile overlay make sure no writeback errors
> have happened since then. And that's where I think we should be using
> new errseq_peek() and errseq_check(&upper_sb->s_wb_err, ofs->errseq)
> infracture. That way we can detect error on upper without consuming
> it upon overlay remount.
> 
> IOW, IMHO, ovl_sync_fs(), should use same mechanism to report error to
> user space both for volatile and non-volatile mounts. And this new
> mechanism of peeking at error without consuming it should be used
> in other paths like remount and possibly other overlay operations(if need
> be). 
> 
> But creating a special path in ovl_sync_fs() for volatile mounts
> only will create conflicts with error reporting for non-volatile
> mounts. And IMHO, these should be same.
> 
> Is there a good reason that why we should treat volatile and non-volatile
> mounts differently in ovl_sync_fs() from error detection and reporting
> point of view.
> 

Fair enough. I'm not that well-versed in overlayfs, so if you see a
better way to do this, then that's fine by me. I just sent this out as a
demonstration of how you could do it. Feel free to drop the second
patch.

I think the simplest solution to most of these issues is to add a new
f_op->syncfs vector. You shouldn't need to propagate errors to the ovl
sb at all if you add that. You can just operate on the upper sb's
s_wb_err, and ignore the one in the ovl sb.

> >  	/*
> >  	 * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC).
> >  	 * All the super blocks will be iterated, including upper_sb.
> > @@ -277,8 +285,6 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
> >  	if (!wait)
> >  		return 0;
> >  
> > 
> > 
> > 
> > -	upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
> > -
> >  	down_read(&upper_sb->s_umount);
> >  	ret = sync_filesystem(upper_sb);
> >  	up_read(&upper_sb->s_umount);
> > @@ -1945,8 +1951,11 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> >  
> > 
> > 
> > 
> >  		sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth;
> >  		sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran;
> > -
> >  	}
> > +
> > +	if (ofs->config.ovl_volatile)
> > +		ofs->errseq = errseq_peek(&ovl_upper_mnt(ofs)->mnt_sb->s_wb_err);
> > +
> >  	oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers);
> >  	err = PTR_ERR(oe);
> >  	if (IS_ERR(oe))
> > -- 
> > 2.29.2
> > 
> 

-- 
Jeff Layton <jlayton@redhat.com>