Re: [PATCH v2 4/4] overlay: Add rudimentary checking of writeback errseq on volatile remount

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Nov 27, 2020 at 01:20:58AM -0800, Sargun Dhillon wrote:
> Volatile remounts validate the following at the moment:
>  * Has the module been reloaded / the system rebooted
>  * Has the workdir been remounted
> 
> This adds a new check for errors detected via the superblock's
> errseq_t. At mount time, the errseq_t is snapshotted to disk,
> and upon remount it's re-verified. This allows for kernel-level
> detection of errors without forcing userspace to perform a
> sync and allows for the hidden detection of writeback errors.
> 
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-unionfs@vger.kernel.org
> Cc: Miklos Szeredi <miklos@szeredi.hu>
> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> ---
>  fs/overlayfs/overlayfs.h | 1 +
>  fs/overlayfs/readdir.c   | 6 ++++++
>  fs/overlayfs/super.c     | 1 +
>  3 files changed, 8 insertions(+)
> 
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index de694ee99d7c..e8a711953b64 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -85,6 +85,7 @@ struct ovl_volatile_info {
>  	 */
>  	uuid_t		ovl_boot_id;	/* Must stay first member */
>  	u64		s_instance_id;
> +	errseq_t	errseq;	/* Implemented as a u32 */
>  } __packed;
>  
>  /*
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 7b66fbb20261..5795b28bb4cf 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -1117,6 +1117,12 @@ static int ovl_verify_volatile_info(struct ovl_fs *ofs,
>  		return -EINVAL;
>  	}
>  
> +	err = errseq_check(&volatiledir->d_sb->s_wb_err, info.errseq);
> +	if (err) {
> +		pr_debug("Workdir filesystem reports errors: %d\n", err);

It probably will make sense to make it pr_err() instead? Will be good
to know if kernel logic detected errors.


Thanks
Vivek