Re: [PATCH 4/4] ovl: alllow remote upper

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Jan 31, 2020 at 12:50:04PM +0100, Miklos Szeredi wrote:
> No reason to prevent upper layer being a remote filesystem.  Do the
> revalidation in that case, just as we already do for lower layers.
> 
> This lets virtiofs be used as upper layer, which appears to be a real use
> case.

Hi Miklos,

I have couple of very basic questions.

- So with this change, we will allow NFS to be upper layer also?

- What does revalidation on lower/upper mean? Does that mean that
  lower/upper can now change underneath overlayfs and overlayfs will
  cope with it. If we still expect underlying layers not to change, then
  what's the point of calling ->revalidate().

Thanks
Vivek

> 
> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> ---
>  fs/overlayfs/namei.c | 3 +--
>  fs/overlayfs/super.c | 8 ++++++--
>  fs/overlayfs/util.c  | 2 ++
>  3 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index 76e61cc27822..0db23baf98e7 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -845,8 +845,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
>  		if (err)
>  			goto out;
>  
> -		if (upperdentry && (upperdentry->d_flags & DCACHE_OP_REAL ||
> -				    unlikely(ovl_dentry_remote(upperdentry)))) {
> +		if (upperdentry && upperdentry->d_flags & DCACHE_OP_REAL) {
>  			dput(upperdentry);
>  			err = -EREMOTE;
>  			goto out;
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 26d4153240a8..ed3a11db9039 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -135,9 +135,14 @@ static int ovl_dentry_revalidate_common(struct dentry *dentry,
>  					unsigned int flags, bool weak)
>  {
>  	struct ovl_entry *oe = dentry->d_fsdata;
> +	struct dentry *upper;
>  	unsigned int i;
>  	int ret = 1;
>  
> +	upper = ovl_dentry_upper(dentry);
> +	if (upper)
> +		ret = ovl_revalidate_real(upper, flags, weak);
> +
>  	for (i = 0; ret > 0 && i < oe->numlower; i++) {
>  		ret = ovl_revalidate_real(oe->lowerstack[i].dentry, flags,
>  					  weak);
> @@ -747,8 +752,7 @@ static int ovl_mount_dir(const char *name, struct path *path)
>  		ovl_unescape(tmp);
>  		err = ovl_mount_dir_noesc(tmp, path);
>  
> -		if (!err && (ovl_dentry_remote(path->dentry) ||
> -			     path->dentry->d_flags & DCACHE_OP_REAL)) {
> +		if (!err && path->dentry->d_flags & DCACHE_OP_REAL) {
>  			pr_err("filesystem on '%s' not supported as upperdir\n",
>  			       tmp);
>  			path_put_init(path);
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 3ad8fb291f7d..c793722739e1 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -96,6 +96,8 @@ void ovl_dentry_update_reval(struct dentry *dentry, struct dentry *upperdentry,
>  	struct ovl_entry *oe = OVL_E(dentry);
>  	unsigned int i, flags = 0;
>  
> +	if (upperdentry)
> +		flags |= upperdentry->d_flags;
>  	for (i = 0; i < oe->numlower; i++)
>  		flags |= oe->lowerstack[i].dentry->d_flags;
>  
> -- 
> 2.21.1
>