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

[Pavel Tatashin <pasha.tatashin@soleen.com>]

On Fri, Mar 26, 2021 at 11:27 AM Tyler Hicks
<tyhicks@linux.microsoft.com> 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.
>
> Fixes: 2522afb86a8c ("libnvdimm/region: Introduce an 'align' attribute")
> Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>

This solves the problem that I had in this thread:
https://lore.kernel.org/lkml/CA+CK2bCD13JBLMxn2mAuRyVQGKBS5ic2UqYSsxXTccszXCmHkA@mail.gmail.com/

Thank you Tyler for root causing and finding a proper fix.

Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>

> ---
>  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;
> +       }
> +
>         /*
>          * 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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