On 2022-08-30 00:45:05, Tyler Hicks wrote:
> 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>
> ---
Friendly ping. I'm hoping this fix can be considered for v6.1. Thanks!
Tyler
>
> 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;
> --
> 2.25.1
>
