Re: [PATCH] libnvdimm/region: Allow setting align attribute on regions without mappings

[Tyler Hicks]

On 2021-03-30 16:32:10, Aneesh Kumar K.V wrote:
> Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx> writes:
> 
> > The alignment constraint for namespace creation in a region was
> > increased, from 2M to 16M, for non-PowerPC architectures in v5.7 with
> > commit 2522afb86a8c ("libnvdimm/region: Introduce an 'align'
> > attribute"). The thought behind the change was that region alignment
> > should be uniform across all architectures and, since PowerPC had the
> > largest alignment constraint of 16M, all architectures should conform to
> > that alignment.
> >
> > The change regressed namespace creation in pre-defined regions that
> > relied on 2M alignment but a workaround was provided in the form of a
> > sysfs attribute, named 'align', that could be adjusted to a non-default
> > alignment value.
> >
> > However, the sysfs attribute's store function returned an error (-ENXIO)
> > when userspace attempted to change the alignment of a region that had no
> > mappings. This affected 2M aligned regions of volatile memory that were
> > defined in a device tree using "pmem-region" and created by the
> > of_pmem_region_driver, since those regions do not contain mappings
> > (ndr_mappings is 0).
> >
> > Allow userspace to set the align attribute on pre-existing regions that
> > do not have mappings so that namespaces can still be within those
> > regions, despite not being aligned to 16M.
> >
> > Fixes: 2522afb86a8c ("libnvdimm/region: Introduce an 'align' attribute")
> > Signed-off-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/nvdimm/region_devs.c | 33 ++++++++++++++++++---------------
> >  1 file changed, 18 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> > index ef23119db574..09cff8aa6b40 100644
> > --- a/drivers/nvdimm/region_devs.c
> > +++ b/drivers/nvdimm/region_devs.c
> > @@ -545,29 +545,32 @@ static ssize_t align_store(struct device *dev,
> >  		struct device_attribute *attr, const char *buf, size_t len)
> >  {
> >  	struct nd_region *nd_region = to_nd_region(dev);
> > -	unsigned long val, dpa;
> > -	u32 remainder;
> > +	unsigned long val;
> >  	int rc;
> >  
> >  	rc = kstrtoul(buf, 0, &val);
> >  	if (rc)
> >  		return rc;
> >  
> > -	if (!nd_region->ndr_mappings)
> > -		return -ENXIO;
> > -
> > -	/*
> > -	 * Ensure space-align is evenly divisible by the region
> > -	 * interleave-width because the kernel typically has no facility
> > -	 * to determine which DIMM(s), dimm-physical-addresses, would
> > -	 * contribute to the tail capacity in system-physical-address
> > -	 * space for the namespace.
> > -	 */
> > -	dpa = div_u64_rem(val, nd_region->ndr_mappings, &remainder);
> > -	if (!is_power_of_2(dpa) || dpa < PAGE_SIZE
> > -			|| val > region_size(nd_region) || remainder)
> > +	if (val > region_size(nd_region))
> >  		return -EINVAL;
> >  
> > +	if (nd_region->ndr_mappings) {
> > +		unsigned long dpa;
> > +		u32 remainder;
> > +
> > +		/*
> > +		 * Ensure space-align is evenly divisible by the region
> > +		 * interleave-width because the kernel typically has no facility
> > +		 * to determine which DIMM(s), dimm-physical-addresses, would
> > +		 * contribute to the tail capacity in system-physical-address
> > +		 * space for the namespace.
> > +		 */
> > +		dpa = div_u64_rem(val, nd_region->ndr_mappings, &remainder);
> > +		if (!is_power_of_2(dpa) || dpa < PAGE_SIZE || remainder)
> > +			return -EINVAL;
> > +	}
> 
> We still want
> 
> else {
> 
> if (!is_power_of_2(val) || val < PAGE_SIZE)
>          return -EINVAL?

Yes, very good point. I'll send out a v2 that ensures val is a power of
2 and at least page size.

Tyler

> 
> }
> > +
> >  	/*
> >  	 * Given that space allocation consults this value multiple
> >  	 * times ensure it does not change for the duration of the
> > -- 
> > 2.25.1
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