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

[Sargun Dhillon <sargun@sargun.me>]

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!