# Re: \[PATCH v2 4/4\] libnvdimm/region: Introduce an 'align' attribute

- List: linuxppc-dev
- 1 message, 1 author
- Page: https://ratatoskr.run/linuxppc-dev/2020/02/5863623/t

## Re: \[PATCH v2 4/4\] libnvdimm/region: Introduce an 'align' attribute

- From: Jeff Moyer \<hidden\>
- Date: 2020-02-14 20:19:21 
- Message: https://ratatoskr.run/linuxppc-dev/2020/02/5863623.md

```text
Dan Williams [ref] writes:

> The align attribute applies an alignment constraint for namespace
> creation in a region. Whereas the 'align' attribute of a namespace
> applied alignment padding via an info block, the 'align' attribute
> applies alignment constraints to the free space allocation.
>
> The default for 'align' is the maximum known memremap_compat_align()
> across all archs (16MiB from PowerPC at time of writing) multiplied by
> the number of interleave ways if there is blk-aliasing. The minimum is
> PAGE_SIZE and allows for the creation of cross-arch incompatible
> namespaces, just as previous kernels allowed, but the expectation is
> cross-arch and mode-independent compatibility by default.
>
> The regression risk with this change is limited to cases that were
> dependent on the ability to create unaligned namespaces, *and* for some
> reason are unable to opt-out of aligned namespaces by writing to
> 'regionX/align'. If such a scenario arises the default can be flipped
> from opt-out to opt-in of compat-aligned namespace creation, but that is
> a last resort. The kernel will otherwise continue to support existing
> defined misaligned namespaces.
>
> Unfortunately this change needs to touch several parts of the
> implementation at once:
>
> - region/available_size: expand busy extents to current align
> - region/max_available_extent: expand busy extents to current align
> - namespace/size: trim free space to current align
>
> ...to keep the free space accounting conforming to the dynamic align
> setting.
>
> Reported-by: Aneesh Kumar K.V <redacted>
> Reported-by: Jeff Moyer <redacted>
> Signed-off-by: Dan Williams <redacted>
> Reviewed-by: Aneesh Kumar K.V <redacted>
> Link: https://lore.kernel.org/r/158041478371.3889308.14542630147672668068.stgit@dwillia2-desk3.amr.corp.intel.com
> Signed-off-by: Dan Williams <redacted>

This looks good to me.

Reviewed-by: Jeff Moyer <redacted>

```