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

[Jeff Layton <jlayton@kernel.org>]

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

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