Re: [PATCH v2] libnvdimm/region: Allow setting align attribute on regions without mappings

[Tyler Hicks <tyhicks@linux.microsoft.com>]

On 2022-09-26 16:18:18, Jeff Moyer wrote:
> Tyler Hicks <tyhicks@linux.microsoft.com> 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.
> >
> > Link: https://lore.kernel.org/lkml/CA+CK2bDJ3hrWoE91L2wpAk+Yu0_=GtYw=4gLDDD7mxs321b_aA@mail.gmail.com
> > Fixes: 2522afb86a8c ("libnvdimm/region: Introduce an 'align' attribute")
> > Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> > ---
> >
> > While testing with a recent kernel release (6.0-rc3), I rediscovered
> > this bug and eventually realized that I never followed through with
> > fixing it upstream. After a year later, here's the v2 that Aneesh
> > requested. Sorry about that!
> >
> > v2:
> > - Included Aneesh's feedback to ensure the val is a power of 2 and
> >   greater than PAGE_SIZE even for regions without mappings
> > - Reused the max_t() trick from default_align() to avoid special
> >   casing, with an if-else, when regions have mappings and when they
> >   don't
> >   + Didn't include Pavel's Reviewed-by since this is a slightly
> >     different approach than what he reviewed in v1
> > - Added a Link commit tag to Pavel's initial problem description
> > v1: https://lore.kernel.org/lkml/20210326152645.85225-1-tyhicks@linux.microsoft.com/
> >
> >  drivers/nvdimm/region_devs.c | 8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> > index 473a71bbd9c9..550ea0bd6c53 100644
> > --- a/drivers/nvdimm/region_devs.c
> > +++ b/drivers/nvdimm/region_devs.c
> > @@ -509,16 +509,13 @@ static ssize_t align_store(struct device *dev,
> >  {
> >  	struct nd_region *nd_region = to_nd_region(dev);
> >  	unsigned long val, dpa;
> > -	u32 remainder;
> > +	u32 mappings, remainder;
> >  	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
> > @@ -526,7 +523,8 @@ static ssize_t align_store(struct device *dev,
> >  	 * contribute to the tail capacity in system-physical-address
> >  	 * space for the namespace.
> >  	 */
> > -	dpa = div_u64_rem(val, nd_region->ndr_mappings, &remainder);
> > +	mappings = max_t(u32, 1, nd_region->ndr_mappings);
> > +	dpa = div_u64_rem(val, mappings, &remainder);
> >  	if (!is_power_of_2(dpa) || dpa < PAGE_SIZE
> >  			|| val > region_size(nd_region) || remainder)
> >  		return -EINVAL;
> 
> The math all looks okay, and this matches what's done in default_align.
> Unfortunately, I don't know enough about the power architecture to
> understand how you can have a region with no dimms (ndr_mappings == 0).

Thanks for having a look!

FWIW, I need this working on arm64. It previously did before the commit
mentioned in the Fixes line. ndr_mappings is 0 when defining a
pmem-region in the device tree. The region is also marked as 'volatile'
but I don't recall if that contributes to ndr_mappings being 0.

Tyler

> 
> -Jeff
>