On Wed, Dec 02, 2020 at 12:02:43PM -0500, Jeff Layton wrote:
[..]
> > > 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;
> +}
Yes, a helper like this should solve the issue at hand. We are not
interested in previous errors. This also sets the ERRSEQ_SEEN on
sample and it will also solve the other issue when after sampling
if error gets seen, we don't want errseq_check() to return error.
Thinking of some possible names for new function.
errseq_sample_seen()
errseq_sample_set_seen()
errseq_sample_consume_unseen()
errseq_sample_current()
Thanks
Vivek
