generic: mount overlayfs r/o if workdir cannot be created

This is the case e.g. if JFFS2 partition is full.

see: https://www.mail-archive.com/linux-unionfs@vger.kernel.org/msg00246.html
(a small error in a logmessage was changed and reported upstream)

runtime tested on ar71xx with kernel 3.18.11 and r45772
this paritially fixes #19564

Signed-off-by: Bastian Bittorf <bittorf@bluebottle.com>
Acked-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 45833

target/linux/ar71xx/patches-3.18/910-overlayfs-fallback-to-readonly-when-full.patch

@@ -0,0 +1,109 @@
+[linux-unionfs added to Cc]
+
+On Tue, May 19, 2015 at 09:51:20AM +0200, Bastian Bittorf wrote:
+> Hi Miklos,
+>
+> sorry for writing directly to you, feel free to forward
+> this to the appropriate mailinglist.
+>
+> we have a problem with mainline overlay filesystem on kernel 3.18:
+> https://dev.openwrt.org/ticket/19564
+>
+> 2 things are odd:
+> when the working filesystem is full, overlays fails with:
+>
+> overlayfs: failed to create directory /overlay/work/work
+>
+> what is strange, that we call it with:
+>
+> mount(overlay, "/mnt", "overlay", MS_NOATIME, lowerdir)
+>
+> see here:
+> http://nbd.name/gitweb.cgi?p=fstools.git;a=blob;f=libfstools/mount.c;h=81176ce399b4cd8e2d347c0008c13dec92407f55;hb=e6004000ff15d7bd32cf5663e8690fc94d7ec747#l125
+>
+> do you have an idea whats wrong?
+> 1) is it really needed, that we need space for creating dir "/overlay/work"?
+> 2) why does overlay need "/overlay/work/work"?
+
+The work directory is needed for atomic copy-up and similar.  It is not actually
+necessary to mount a read-only overlay.  Post 4.0 it is possible to mount the
+overlay without workdir (but even then it won't happen automatically in case the
+upper fs is full, so this should be fixed in the latest kernel too).
+
+Could you please try the following patch?  If the workdir can't be created it
+will fall back to mounting the overlay read-only.
+
+Thanks,
+Miklos
+
+---
+ fs/overlayfs/copy_up.c |    3 +++
+ fs/overlayfs/dir.c     |    9 +++++++++
+ fs/overlayfs/super.c   |   12 +++++++++---
+ 3 files changed, 21 insertions(+), 3 deletions(-)
+
+--- a/fs/overlayfs/copy_up.c
++++ b/fs/overlayfs/copy_up.c
+@@ -300,6 +300,9 @@ int ovl_copy_up_one(struct dentry *paren
+	struct cred *override_cred;
+	char *link = NULL;
+
++	if (WARN_ON(!workdir))
++		return -EROFS;
++
+	ovl_path_upper(parent, &parentpath);
+	upperdir = parentpath.dentry;
+
+--- a/fs/overlayfs/dir.c
++++ b/fs/overlayfs/dir.c
+@@ -222,6 +222,9 @@ static struct dentry *ovl_clear_empty(st
+	struct kstat stat;
+	int err;
+
++	if (WARN_ON(!workdir))
++		return ERR_PTR(-EROFS);
++
+	err = ovl_lock_rename_workdir(workdir, upperdir);
+	if (err)
+		goto out;
+@@ -322,6 +325,9 @@ static int ovl_create_over_whiteout(stru
+	struct dentry *newdentry;
+	int err;
+
++	if (WARN_ON(!workdir))
++		return -EROFS;
++
+	err = ovl_lock_rename_workdir(workdir, upperdir);
+	if (err)
+		goto out;
+@@ -506,6 +512,9 @@ static int ovl_remove_and_whiteout(struc
+	struct dentry *opaquedir = NULL;
+	int err;
+
++	if (WARN_ON(!workdir))
++		return -EROFS;
++
+	if (is_dir) {
+		opaquedir = ovl_check_empty_and_clear(dentry);
+		err = PTR_ERR(opaquedir);
+--- a/fs/overlayfs/super.c
++++ b/fs/overlayfs/super.c
+@@ -740,9 +740,15 @@ static int ovl_fill_super(struct super_b
+	ufs->workdir = ovl_workdir_create(ufs->upper_mnt, workpath.dentry);
+	err = PTR_ERR(ufs->workdir);
+	if (IS_ERR(ufs->workdir)) {
+-		pr_err("overlayfs: failed to create directory %s/%s\n",
+-		       ufs->config.workdir, OVL_WORKDIR_NAME);
+-		goto out_put_lower_mnt;
++		if (err == -ENOSPC || err == -EROFS) {
++			pr_warning("overlayfs: failed to create work directory (%s), mounting read-only\n", err == ENOSPC ? "ENOSPC" : "EROFS");
++			sb->s_flags |= MS_RDONLY;
++			ufs->workdir = NULL;
++		} else {
++			pr_err("overlayfs: failed to create directory %s/%s\n",
++			       ufs->config.workdir, OVL_WORKDIR_NAME);
++			goto out_put_lower_mnt;
++		}
+	}
+
+	/*