On Mon, 2020-12-14 at 18:53 -0500, Jeff Layton wrote:
> On Mon, 2020-12-14 at 16:38 -0500, Vivek Goyal wrote:
> > On Sun, Dec 13, 2020 at 08:27:13AM -0500, Jeff Layton wrote:
> > > Peek at the upper layer's errseq_t at mount time for volatile mounts,
> > > and record it in the per-sb info. In sync_fs, check for an error since
> > > the recorded point and set it in the overlayfs superblock if there was
> > > one.
> > >
> > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > ---
> >
> > While we are solving problem for non-volatile overlay mount, I also
> > started thinking, what about non-volatile overlay syncfs() writeback errors.
> > Looks like these will not be reported to user space at all as of now
> > (because we never update overlay_sb->s_wb_err ever).
> >
> > A patch like this might fix it. (compile tested only).
> >
> > overlayfs: Report syncfs() errors to user space
> >
> > Currently, syncfs(), calls filesystem ->sync_fs() method but ignores the
> > return code. But certain writeback errors can still be reported on
> > syncfs() by checking errors on super block.
> >
> > ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
> >
> > For the case of overlayfs, we never set overlayfs super block s_wb_err. That
> > means sync() will never report writeback errors on overlayfs uppon syncfs().
> >
> > Fix this by updating overlay sb->sb_wb_err upon ->sync_fs() call. And that
> > should mean that user space syncfs() call should see writeback errors.
> >
> > ovl_fsync() does not need anything special because if there are writeback
> > errors underlying filesystem will report it through vfs_fsync_range() return
> > code and user space will see it.
> >
> > Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> > ---
> > fs/overlayfs/ovl_entry.h | 1 +
> > fs/overlayfs/super.c | 14 +++++++++++---
> > 2 files changed, 12 insertions(+), 3 deletions(-)
> >
> > Index: redhat-linux/fs/overlayfs/super.c
> > ===================================================================
> > --- redhat-linux.orig/fs/overlayfs/super.c 2020-12-14 15:33:43.934400880 -0500
> > +++ redhat-linux/fs/overlayfs/super.c 2020-12-14 16:15:07.127400880 -0500
> > @@ -259,7 +259,7 @@ static int ovl_sync_fs(struct super_bloc
> > {
> > struct ovl_fs *ofs = sb->s_fs_info;
> > struct super_block *upper_sb;
> > - int ret;
> > + int ret, ret2;
> >
> >
> >
> >
> > if (!ovl_upper_mnt(ofs))
> > return 0;
> > @@ -283,7 +283,14 @@ static int ovl_sync_fs(struct super_bloc
> > ret = sync_filesystem(upper_sb);
> > up_read(&upper_sb->s_umount);
> >
> >
> >
> >
> > - return ret;
> > + if (errseq_check(&upper_sb->s_wb_err, sb->s_wb_err)) {
> > + /* Upper sb has errors since last time */
> > + spin_lock(&ofs->errseq_lock);
> > + ret2 = errseq_check_and_advance(&upper_sb->s_wb_err,
> > + &sb->s_wb_err);
> > + spin_unlock(&ofs->errseq_lock);
> > + }
> > + return ret ? ret : ret2;
>
> I think this is probably not quite right.
>
> The problem I think is that the SEEN flag is always going to end up
> being set in sb->s_wb_err, and that is going to violate the desired
> semantics. If the writeback error occurred after all fd's were closed,
> then the next opener wouldn't see it and you'd lose the error.
>
> We probably need a function to cleanly propagate the error from one
> errseq_t to another so that that doesn't occur. I'll have to think about
> it.
>
So, the problem is that we can't guarantee that we'll have an open file
when sync_fs is called. So if you do the check_and_advance in the
context of a sync() syscall, you'll effectively ensure that a later
opener on the upper layer won't see the error (since the upper_sb's
errseq_t will be marked SEEN.
It's not clear to me what semantics you want in the following situation:
mount upper layer
mount overlayfs with non-volatile upper layer
do "stuff" on overlayfs, and close all files on overlayfs
get a writeback error on upper layer
call sync() (sync_fs gets run)
open file on upper layer mount
call syncfs() on upper-layer fd
Should that last syncfs error report an error?
Also, suppose if at the end we instead opened a file on overlayfs and
issued the syncfs() there -- should we see the error in that case?
> > }
> >
> >
> >
> >
> > /**
> > @@ -1873,6 +1880,7 @@ static int ovl_fill_super(struct super_b
> > if (!cred)
> > goto out_err;
> >
> >
> >
> >
> > + spin_lock_init(&ofs->errseq_lock);
> > /* Is there a reason anyone would want not to share whiteouts? */
> > ofs->share_whiteout = true;
> >
> >
> >
> >
> > @@ -1945,7 +1953,7 @@ static int ovl_fill_super(struct super_b
> >
> >
> >
> >
> > 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_wb_err = errseq_sample(&ovl_upper_mnt(ofs)->mnt_sb->s_wb_err);
> > }
> > oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers);
> > err = PTR_ERR(oe);
> > Index: redhat-linux/fs/overlayfs/ovl_entry.h
> > ===================================================================
> > --- redhat-linux.orig/fs/overlayfs/ovl_entry.h 2020-12-14 15:33:43.934400880 -0500
> > +++ redhat-linux/fs/overlayfs/ovl_entry.h 2020-12-14 15:34:13.509400880 -0500
> > @@ -79,6 +79,7 @@ struct ovl_fs {
> > atomic_long_t last_ino;
> > /* Whiteout dentry cache */
> > struct dentry *whiteout;
> > + spinlock_t errseq_lock;
> > };
> >
> >
> >
> >
> > static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs)
> >
>
--
Jeff Layton <jlayton@xxxxxxxxxx>
