Re: [PATCH] vfs, syncfs: Do not ignore return code from ->sync_fs()

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Dec 16, 2020 at 09:57:49AM -0500, Jeff Layton wrote:
> On Wed, 2020-12-16 at 09:38 -0500, Vivek Goyal wrote:
> > I see that current implementation of __sync_filesystem() ignores the
> > return code from ->sync_fs(). I am not sure why that's the case.
> > 
> > Ignoring ->sync_fs() return code is problematic for overlayfs where
> > it can return error if sync_filesystem() on upper super block failed.
> > That error will simply be lost and sycnfs(overlay_fd), will get
> > success (despite the fact it failed).
> > 
> > I am assuming that we want to continue to call __sync_blockdev()
> > despite the fact that there have been errors reported from
> > ->sync_fs(). So I wrote this simple patch which captures the
> > error from ->sync_fs() but continues to call __sync_blockdev()
> > and returns error from sync_fs() if there is one.
> > 
> > There might be some very good reasons to not capture ->sync_fs()
> > return code, I don't know. Hence thought of proposing this patch.
> > Atleast I will get to know the reason. I still need to figure
> > a way out how to propagate overlay sync_fs() errors to user
> > space.
> > 
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > ---
> >  fs/sync.c |    8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > Index: redhat-linux/fs/sync.c
> > ===================================================================
> > --- redhat-linux.orig/fs/sync.c	2020-12-16 09:15:49.831565653 -0500
> > +++ redhat-linux/fs/sync.c	2020-12-16 09:23:42.499853207 -0500
> > @@ -30,14 +30,18 @@
> >   */
> >  static int __sync_filesystem(struct super_block *sb, int wait)
> >  {
> > +	int ret, ret2;
> > +
> >  	if (wait)
> >  		sync_inodes_sb(sb);
> >  	else
> >  		writeback_inodes_sb(sb, WB_REASON_SYNC);
> >  
> > 
> >  	if (sb->s_op->sync_fs)
> > -		sb->s_op->sync_fs(sb, wait);
> > -	return __sync_blockdev(sb->s_bdev, wait);
> > +		ret = sb->s_op->sync_fs(sb, wait);
> > +	ret2 = __sync_blockdev(sb->s_bdev, wait);
> > +
> > +	return ret ? ret : ret2;
> >  }
> >  
> > 
> >  /*
> > 
> 
> I posted a patchset that took a similar approach a couple of years ago,
> and we decided not to go with it [1].
> 
> While it's not ideal to ignore the error here, I think this is likely to
> break stuff.

So one side affect I see is that syncfs() might start returning errors
in some cases which were not reported at all. I am wondering will that
count as breakage.

> What may be better is to just make sync_fs void return, so
> people don't think that returned errors there mean anything.

May be. 

But then question remains that how do we return error to user space
in syncfs(fd) for overlayfs. I will not be surprised if other
filesystems want to return errors as well.

Shall I create new helpers and call these in case of syncfs(). But
that too will start returning new errors on syncfs(). So it does
not solve that problem (if it is a problem).

Or we can define a new super block op say ->sync_fs2() and call that
first and in that case capture return code. That way it will not
impact existing cases and overlayfs can possibly make use of
->sync_fs2() and return error. IOW, impact will be limited to
only file systems which chose to implement ->sync_fs2().

Thanks
Vivek

> 
> [1]: https://lore.kernel.org/linux-fsdevel/20180518123415.28181-1-jlayton@kernel.org/
> -- 
> Jeff Layton <jlayton@kernel.org>
>