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

[Jeff Layton <jlayton@redhat.com>]

On Wed, 2020-12-02 at 12:29 -0500, Vivek Goyal wrote:
> 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()
> 

errseq_sample_consume_unseen() sounds good, though maybe it should be
"ignore_unseen"? IDK, naming this stuff is the hardest part.

If you don't want to add a new helper, I think you'd probably also be
able to do something like this in fill_super:

    errseq_sample()
    errseq_check_and_advance()


...and just ignore the error returned by the check and advance. At that
point, the cursor should be caught up and any subsequent syncfs call
should return 0 until you record another error. It's a little less
efficient, but only slightly so.
-- 
Jeff Layton <jlayton@redhat.com>