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

[Jeff Layton <jlayton@redhat.com>]

On Wed, 2020-12-02 at 10:07 -0500, Vivek Goyal wrote:
> On Wed, Dec 02, 2020 at 01:27:20AM -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.
> > 
> > We decided[1] that 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 occured,
> > 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.
> > 
> > [1]: https://lore.kernel.org/linux-fsdevel/36d820394c3e7cd1faa1b28a8135136d5001dadd.camel@redhat.com/T/#u
> > 
> > Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> > Suggested-by: Amir Goldstein <amir73il@gmail.com>
> > Cc: linux-fsdevel@vger.kernel.org
> > Cc: linux-unionfs@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>
> > ---
> >  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                    | 24 +++++++++++++++-------
> >  fs/overlayfs/util.c                     | 27 +++++++++++++++++++++++++
> >  7 files changed, 61 insertions(+), 11 deletions(-)
> > 
> > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
> > index 580ab9a0fe31..3af569cea6a7 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 error occurs on the upperdir's filesystem after a volatile mount
> > +takes place, all sync functions will return the last error observed on the
> > +upperdir filesystem.  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 802259f33c28..2479b297a966 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_check_sync(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..af79c3a2392e 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_check_sync(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..9460a52abea3 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;
> > +	/* snapshot of upperdir's errseq_t at mount time for volatile mounts */
> > +	errseq_t upper_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..f7f1a29e290f 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_check_sync(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..82a096a05bce 100644
> > --- a/fs/overlayfs/super.c
> > +++ b/fs/overlayfs/super.c
> > @@ -261,11 +261,18 @@ 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_check_sync(ofs);
> > +	/*
> > +	 * We have to always set the err, because the return value isn't
> > +	 * checked, and instead VFS looks at the writeback errseq after
> > +	 * this call.
> > +	 */
> > +	if (ret < 0)
> > +		errseq_set(&sb->s_wb_err, ret);
> 
> I was wondering that why errseq_set() will result in returning error
> all the time. Then realized that last syncfs() call must have set
> ERRSEQ_SEEN flag and that will mean errseq_set() will increment
> counter and that means this syncfs() will will return error too. Cool.
> 
> > +
> > +	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 +1934,8 @@ 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_mnt_sb;
> > +
> >  		if (!ofs->config.workdir) {
> >  			pr_err("missing 'workdir'\n");
> >  			goto out_err;
> > @@ -1943,9 +1952,10 @@ 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;
> > -
> > +		upper_mnt_sb = ovl_upper_mnt(ofs)->mnt_sb;
> > +		sb->s_stack_depth = upper_mnt_sb->s_stack_depth;
> > +		sb->s_time_gran = upper_mnt_sb->s_time_gran;
> > +		ofs->upper_errseq = errseq_sample(&upper_mnt_sb->s_wb_err);
> 
> I asked this question in last email as well. errseq_sample() will return
> 0 if current error has not been seen yet. That means next time a sync
> call comes for volatile mount, it will return an error. But that's
> not what we want. When we mounted a volatile overlay, if there is an
> existing error (seen/unseen), we don't care. We only care if there
> is a new error after the volatile mount, right?
> 
> I guess we will need another helper similar to errseq_smaple() which
> just returns existing value of errseq. And then we will have to
> do something about errseq_check() to not return an error if "since"
> and "eseq" differ only by "seen" bit.
> 
> Otherwise in current form, volatile mount will always return error
> if upperdir has error and it has not been seen by anybody.
> 
> How did you finally end up testing the error case. Want to simualate
> error aritificially and test it.
> 

If you don't want to see errors that occurred before you did the mount,
then you probably can just resurrect and rename the original version of
errseq_sample. Something like this, but with a different name:

+errseq_t errseq_sample(errseq_t *eseq)
+{
+       errseq_t old = READ_ONCE(*eseq);
+       errseq_t new = old;
+
+       /*
+        * For the common case of no errors ever having been set, we can skip
+        * marking the SEEN bit. Once an error has been set, the value will
+        * never go back to zero.
+        */
+       if (old != 0) {
+               new |= ERRSEQ_SEEN;
+               if (old != new)
+                       cmpxchg(eseq, old, new);
+       }
+       return new;
+}


>  	}
>  	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..9b460cd7b151 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_check_sync provides sync checking, and safety for volatile mounts
> + *
> + * Returns 1 if sync 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_check_sync(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->upper_errseq);

I guess we can do another patch later to set one global flag in overlayfs
super block and use that flag to return errors on other paths like 
read/write etc. But that's for a separate patch later.

Thanks
Vivek


-- 
Jeff Layton <jlayton@redhat.com>