Re: [PATCH v5 2/2] dax/kmem: allow kmem to add memory with memmap_on_memory

[Verma, Vishal L]

On Thu, 2023-10-05 at 14:16 -0700, Dan Williams wrote:
> Vishal Verma wrote:
> > 
<..>

> > +
> > +       rc = kstrtobool(buf, &val);
> > +       if (rc)
> > +               return rc;
> 
> Perhaps:
> 
> if (dev_dax->memmap_on_memory == val)
>         return len;
> 
> ...and skip the check below when it is going to be a nop
> 
> > +
> > +       device_lock(dax_region->dev);
> > +       if (!dax_region->dev->driver) {
> 
> Is the polarity backwards here? I.e. if the device is already attached to
> the kmem driver it is too late to modify memmap_on_memory policy.

Hm this sounded logical until I tried it. After a reconfigure-device to
devdax (i.e. detach kmem), I get the -EBUSY if I invert this check.

> 
> > +               device_unlock(dax_region->dev);
> > +               return -ENXIO;
> 
> I would expect -EBUSY since disabling the device allows the property to be
> set and -ENXIO implies a more permanent error.
> 
> > +       }
> > +
> > +       dev_dax->memmap_on_memory = val;
> > +
> > +       device_unlock(dax_region->dev);
> > +       return len;
> > +}
> > +static DEVICE_ATTR_RW(memmap_on_memory);
> 
> This new attribute needs a new Documentation/ABI/ entry... in fact all of
> these attributes need Documentation/ entries. I can help with that base
> document to get things started.
> 
> Perhaps split this sysfs ABI into its own patch and, depending on how fast
> we can pull the Documentation together, start with the
> region-driver-conveyed approach in the meantime.

Yep I'll split this out and I can do a separate series to add the ABI
docs for /sys/bus/dax, and include this new ABI in that as well.

Agreed with all other comments.