Tyler Hicks wrote:
> On 2022-09-26 16:18:18, Jeff Moyer wrote:
> > Tyler Hicks <tyhi...@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...@mail.gmail.com
> > > Fixes: 2522afb86a8c ("libnvdimm/region: Introduce an 'align' attribute")
> > > Signed-off-by: Tyler Hicks <tyhi...@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-tyhi...@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.
Oh, ndr_mappings being 0 can also happen for cases where the platform
just does not advertise any DIMMs / nmemX devices that map to a region.
It is not possible for ndr_mappings to change at run time as they are
only established at nd_region_create() time. The change looks good to
me.
