Re: [PATCH 05/18] xfs: separate out initial attr_set states

[Dave Chinner <david@fromorbit.com>]

On Wed, May 11, 2022 at 08:39:52AM -0700, Darrick J. Wong wrote:
> On Wed, May 11, 2022 at 06:35:13PM +1000, Dave Chinner wrote:
> > On Wed, May 11, 2022 at 11:38:51AM +1000, Dave Chinner wrote:
> > > On Tue, May 10, 2022 at 06:08:48PM -0700, Darrick J. Wong wrote:
> > > > On Wed, May 11, 2022 at 11:06:51AM +1000, Dave Chinner wrote:
> > > > > I'm going to leave this for the moment (cleanup note made) because I
> > > > > don't want to have to find out that I missed a corner case somewhere
> > > > > they hard way right now. It's basically there to stop log recovery
> > > > > crashing hard, which only occurs when the experimental larp code is
> > > > > running, so I think this is safe to leave for a later cleanup.
> > > > 
> > > > Hmm, in that case, can this become:
> > > > 
> > > > 	if (!args->dp->i_afp) {
> > > > 		ASSERT(0);
> > > > 		return XFS_DAS_DONE;
> > > > 	}
> > > 
> > > OK.
> > 
> > Ok, now generic/051 has reminded me exactly what this was for.
> > 
> > Shortform attr remove will remove the attr and the attr fork from
> > this code:
> > 
> >         case XFS_DAS_SF_REMOVE:                                                  
> >                 error = xfs_attr_sf_removename(args);                            
> >                 attr->xattri_dela_state = xfs_attr_complete_op(attr,             
> >                                                 xfs_attr_init_add_state(args));  
> >                 break;                                                           
> > 
> > But if we are doing this as part of a REPLACE operation and we
> > still need to add the new attr, it calls xfs_attr_init_add_state()
> > to get the add state we should start with. That then hits the
> > null args->dp->i_afp case because the fork got removed.
> > 
> > This can't happen if we are doing a replace op, so we'd then check
> > if it's a shortform attr fork and return XFS_DAS_SF_ADD for the
> > replace to then execute. But it's not a replace op, so we can
> > have a null attr fork.
> > 
> > I'm going to restore the old code with a comment so that I don't
> > forget this again.
> > 
> > /*
> >  * If called from the completion of a attr remove to determine
> >  * the next state, the attribute fork may be null. This can occur on
> >  * a pure remove, but we grab the next state before we check if a
> >  * replace operation is being performed. Hence if the attr fork is
> >  * null, it's a pure remove operation and we are done.
> >  */
> 
> Ahh, I see -- sf_removename will /never/ kill i_afp if we're doing a
> DA_OP_REPLACE or ADDNAME, and leaf_removename also won't do that if
> we're doing DA_OP_REPLACE.  IOWs, only a removexattr operation can
> result in i_afp being freed.
> 
> And the XATTR_CREATE operation always guarantee that i_afp is non-null
> before we start, so xfs_attr_defer_add should never be called with
> args->dp->i_afp == NULL, hence it'll never hit that state.
> 
> Would you mind adding a sentence to the comment?
> 
> "A pure create ensures the existence of i_afp and never encounters this
> state."

Sure.

-Dave.
-- 
Dave Chinner
david@fromorbit.com