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

[Jeff Moyer]

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.
>
> Link: https://lore.kernel.org/lkml/CA+CK2bDJ3hrWoE91L2wpAk+Yu0_=GtYw=4gLDDD7mxs321b_aA@xxxxxxxxxxxxxx
> Fixes: 2522afb86a8c ("libnvdimm/region: Introduce an 'align' attribute")
> Signed-off-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>
> ---
>
> 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@xxxxxxxxxxxxxxxxxxx/
>
>  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).

-Jeff