Re: [PATCH v4] overlay: Implement volatile-specific fsync error behaviour

On Tue, Jan 19, 2021 at 06:49:36AM -0500, Jeff Layton wrote:

On Thu, 2021-01-07 at 16:10 -0800, Sargun Dhillon wrote:

...

Overlayfs's volatile option allows the user to bypass all forced sync calls to the upperdir filesystem. This comes at the cost of safety. We can never ensure that the user's data is intact, but we can make a best effort to expose whether or not the data is likely to be in a bad state.

The best way to handle this in the time being is that if an overlayfs's upperdir experiences an error after a volatile mount occurs, that error will be returned on fsync, fdatasync, sync, and syncfs. This is contradictory to the traditional behaviour of VFS which fails the call once, and only raises an error if a subsequent fsync error has occurred, and been raised by the filesystem.

One awkward aspect of the patch is that we have to manually set the superblock's errseq_t after the sync_fs callback as opposed to just returning an error from syncfs. This is because the call chain looks something like this:

sys_syncfs -> sync_filesystem -> __sync_filesystem -> /* The return value is ignored here sb->s_op->sync_fs(sb) _sync_blockdev /* Where the VFS fetches the error to raise to userspace */ errseq_check_and_advance

Because of this we call errseq_set every time the sync_fs callback occurs. Due to the nature of this seen / unseen dichotomy, if the upperdir is an inconsistent state at the initial mount time, overlayfs will refuse to mount, as overlayfs cannot get a snapshot of the upperdir's errseq that will increment on error until the user calls syncfs.

Signed-off-by: Sargun Dhillon sargun@sargun.me Suggested-by: Amir Goldstein amir73il@gmail.com Reviewed-by: Amir Goldstein amir73il@gmail.com Fixes: c86243b090bc ("ovl: provide a mount option "volatile"") Cc: linux-fsdevel@vger.kernel.org Cc: linux-unionfs@vger.kernel.org Cc: stable@vger.kernel.org Cc: Jeff Layton jlayton@redhat.com Cc: Miklos Szeredi miklos@szeredi.hu Cc: Amir Goldstein amir73il@gmail.com Cc: Vivek Goyal vgoyal@redhat.com Cc: Matthew Wilcox willy@infradead.org

---

Documentation/filesystems/overlayfs.rst | 8 ++++++  fs/overlayfs/file.c | 5 ++--  fs/overlayfs/overlayfs.h | 1 +  fs/overlayfs/ovl_entry.h | 2 ++  fs/overlayfs/readdir.c | 5 ++--  fs/overlayfs/super.c | 34 ++++++++++++++++++++-----  fs/overlayfs/util.c | 27 ++++++++++++++++++++  7 files changed, 71 insertions(+), 11 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst index 580ab9a0fe31..137afeb3f581 100644 --- a/Documentation/filesystems/overlayfs.rst +++ b/Documentation/filesystems/overlayfs.rst @@ -575,6 +575,14 @@ without significant effort.  The advantage of mounting with the "volatile" option is that all forms of  sync calls to the upper filesystem are omitted.

+In order to avoid a giving a false sense of safety, the syncfs (and fsync) +semantics of volatile mounts are slightly different than that of the rest of +VFS. If any writeback error occurs on the upperdir's filesystem after a +volatile mount takes place, all sync functions will return an error. Once this +condition is reached, the filesystem will not recover, and every subsequent sync +call will return an error, even if the upperdir has not experience a new error +since the last sync call.

When overlay is mounted with "volatile" option, the directory  "$workdir/work/incompat/volatile" is created. During next mount, overlay  checks for this directory and refuses to mount if present. This is a strong diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c index a1f72ac053e5..5c5c3972ebd0 100644 --- a/fs/overlayfs/file.c +++ b/fs/overlayfs/file.c @@ -445,8 +445,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)   const struct cred *old_cred;   int ret;

• if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb)))

• return 0;
  

• ret = ovl_sync_status(OVL_FS(file_inode(file)->i_sb));
• if (ret <= 0)

• return ret;
  

ret = ovl_real_fdget_meta(file, &real, !datasync);   if (ret) diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h index f8880aa2ba0e..9f7af98ae200 100644 --- a/fs/overlayfs/overlayfs.h +++ b/fs/overlayfs/overlayfs.h @@ -322,6 +322,7 @@ int ovl_check_metacopy_xattr(struct ovl_fs *ofs, struct dentry *dentry);  bool ovl_is_metacopy_dentry(struct dentry *dentry);  char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry,   int padding); +int ovl_sync_status(struct ovl_fs *ofs);

static inline bool ovl_is_impuredir(struct super_block *sb,   struct dentry *dentry) diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h index 1b5a2094df8e..b208eba5d0b6 100644 --- a/fs/overlayfs/ovl_entry.h +++ b/fs/overlayfs/ovl_entry.h @@ -79,6 +79,8 @@ struct ovl_fs {   atomic_long_t last_ino;   /* Whiteout dentry cache */   struct dentry *whiteout;

• /* r/o snapshot of upperdir sb's only taken on volatile mounts */
• errseq_t errseq;

};

static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs) diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c index 01620ebae1bd..a273ef901e57 100644 --- a/fs/overlayfs/readdir.c +++ b/fs/overlayfs/readdir.c @@ -909,8 +909,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,   struct file *realfile;   int err;

• if (!ovl_should_sync(OVL_FS(file->f_path.dentry->d_sb)))

• return 0;
  

• err = ovl_sync_status(OVL_FS(file->f_path.dentry->d_sb));
• if (err <= 0)

• return err;
  

realfile = ovl_dir_real_file(file, true);   err = PTR_ERR_OR_ZERO(realfile); diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c index 290983bcfbb3..d23177a53c95 100644 --- a/fs/overlayfs/super.c +++ b/fs/overlayfs/super.c @@ -261,11 +261,20 @@ static int ovl_sync_fs(struct super_block *sb, int wait)   struct super_block *upper_sb;   int ret;

• if (!ovl_upper_mnt(ofs))

• return 0;
  

• ret = ovl_sync_status(ofs);
• /*

• * We have to always set the err, because the return value isn't
  

• * checked in syncfs, and instead indirectly return an error via
  

• * the sb's writeback errseq, which VFS inspects after this call.
  

• */
  

• if (ret < 0) {

• errseq_set(&sb->s_wb_err, -EIO);
  

• return -EIO;
  

• }

Apologies for the late review:

You're converting the error you got back from errseq_check to -EIO unconditionally here. Why? Would it not be better to just pass the error through? Like this:

I believe Vivek brought this up as a potentially confusing issue. For example, you can end up with an ENOSPC on a write, and then delete a file, and still get ENOSPC. It's just easier to return EIO, given there's not really anything the user can do to recover the system.

if (ret < 0) { errseq_set(&sb->s_wb_err, ret); return ret; }

?

...

• if (!ret)

• return ret;
  

• if (!ovl_should_sync(ofs))

• return 0;
  

/*   * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC).   * All the super blocks will be iterated, including upper_sb. @@ -1927,6 +1936,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)   sb->s_op = &ovl_super_operations;

if (ofs->config.upperdir) {

• struct super_block *upper_sb;
  

if (!ofs->config.workdir) {   pr_err("missing 'workdir'\n");   goto out_err; @@ -1936,6 +1947,16 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)   if (err)   goto out_err;

• upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
  

• if (!ovl_should_sync(ofs)) {
  

• 	ofs->errseq = errseq_sample(&upper_sb->s_wb_err);
  

• 	if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) {
  

• 		err = -EIO;
  

• 		pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n");
  

• 		goto out_err;
  

• 	}
  

• }
  

err = ovl_get_workdir(sb, ofs, &upperpath);   if (err)   goto out_err; @@ -1943,9 +1964,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)   if (!ofs->workdir)   sb->s_flags |= SB_RDONLY;

• sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth;
  

• sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran;
  

• sb->s_stack_depth = upper_sb->s_stack_depth;
  

• sb->s_time_gran = upper_sb->s_time_gran;
  

}   oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers);   err = PTR_ERR(oe); diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c index 23f475627d07..6e7b8c882045 100644 --- a/fs/overlayfs/util.c +++ b/fs/overlayfs/util.c @@ -950,3 +950,30 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry,   kfree(buf);   return ERR_PTR(res);  }

+/*

• • ovl_sync_status() - Check fs sync status for volatile mounts
• • Returns 1 if this is not a volatile mount and a real sync is required.
• • Returns 0 if syncing can be skipped because mount is volatile, and no errors
• • have occurred on the upperdir since the mount.
• • Returns -errno if it is a volatile mount, and the error that occurred since
• • the last mount. If the error code changes, it'll return the latest error
• • code.
• */

+int ovl_sync_status(struct ovl_fs *ofs) +{

• struct vfsmount *mnt;
• if (ovl_should_sync(ofs))

• return 1;
  

• mnt = ovl_upper_mnt(ofs);
• if (!mnt)

• return 0;
  

• return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->errseq);

+}

Aside from my minor question about converting the error unconditionally, this all looks fine. You can add: Reviewed-by: Jeff Layton jlayton@kernel.org

Nice work!

Thanks!