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From:   Jeff Moyer <jmoyer@redhat.com>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     linux-nvdimm@lists.01.org,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        vishal.l.verma@intel.com, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2 4/4] libnvdimm/region: Introduce an 'align' attribute
References: <158155489850.3343782.2687127373754434980.stgit@dwillia2-desk3.amr.corp.intel.com>
        <158155491952.3343782.4541070487858304628.stgit@dwillia2-desk3.amr.corp.intel.com>
Date:   Fri, 14 Feb 2020 15:19:07 -0500
In-Reply-To: <158155491952.3343782.4541070487858304628.stgit@dwillia2-desk3.amr.corp.intel.com>
        (Dan Williams's message of "Wed, 12 Feb 2020 16:48:39 -0800")
Message-ID: <x495zg8exc4.fsf@segfault.boston.devel.redhat.com>
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Dan Williams <dan.j.williams@intel.com> 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@linux.ibm.com>
> Reported-by: Jeff Moyer <jmoyer@redhat.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Link: https://lore.kernel.org/r/158041478371.3889308.14542630147672668068.stgit@dwillia2-desk3.amr.corp.intel.com
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

This looks good to me.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>