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

[Jeff Moyer]

Dan Williams <dan.j.williams@xxxxxxxxx> 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 <aneesh.kumar@xxxxxxxxxxxxx>
> Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/158041478371.3889308.14542630147672668068.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

This looks good to me.

Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>