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From: Jeff Moyer <jmoyer@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-nvdimm <linux-nvdimm@lists.01.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v2 1/4] mm/memremap_pages: Introduce memremap_compat_align()
Date: Fri, 14 Feb 2020 15:59:08 -0500
Message-ID: <x49k14odgwz.fsf@segfault.boston.devel.redhat.com> (raw)
In-Reply-To: <CAPcyv4i8xNEsdX=8c2+ehf24U2AFcc-sKmAPS9UoVvm8z0aRng@mail.gmail.com>

Dan Williams <dan.j.williams@intel.com> writes:

> On Thu, Feb 13, 2020 at 8:58 AM Jeff Moyer <jmoyer@redhat.com> wrote:

>> I have just a couple of questions.
>>
>> First, can you please add a comment above the generic implementation of
>> memremap_compat_align describing its purpose, and why a platform might
>> want to override it?
>
> Sure, how about:
>
> /*
>  * The memremap() and memremap_pages() interfaces are alternately used
>  * to map persistent memory namespaces. These interfaces place different
>  * constraints on the alignment and size of the mapping (namespace).
>  * memremap() can map individual PAGE_SIZE pages. memremap_pages() can
>  * only map subsections (2MB), and at least one architecture (PowerPC)
>  * the minimum mapping granularity of memremap_pages() is 16MB.
>  *
>  * The role of memremap_compat_align() is to communicate the minimum
>  * arch supported alignment of a namespace such that it can freely
>  * switch modes without violating the arch constraint. Namely, do not
>  * allow a namespace to be PAGE_SIZE aligned since that namespace may be
>  * reconfigured into a mode that requires SUBSECTION_SIZE alignment.
>  */

Well, if we modify the x86 variant to be PAGE_SIZE, I think that text
won't work.  How about:

/*
 * memremap_compat_align should return the minimum alignment for
 * mapping memory via memremap() and memremap_pages().  For x86, this
 * is the system PAGE_SIZE.  Other architectures may impose different
 * restrictions, as is seen on powerpc where the minimum alignment is
 * tied to the linear mapping page size.
 *
 * When creating persistent memory namespaces, the alignment is forced
 * to the least common denominator (MEMREMAP_COMPAT_ALIGN_MAX,
 * currently 16MB).  However, older kernels did not enforce this
 * behavior, so we allow mapping namespaces with smaller alignments,
 * so long as the platform supports it.  See nvdimm_namespace_common_probe.
 */

-Jeff
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  parent reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-13  0:48 [PATCH v2 0/4] libnvdimm: Cross-arch compatible namespace alignment Dan Williams
2020-02-13  0:48 ` [PATCH v2 1/4] mm/memremap_pages: Introduce memremap_compat_align() Dan Williams
2020-02-13 16:57   ` Jeff Moyer
2020-02-13 18:26     ` Dan Williams
2020-02-14  3:26       ` Aneesh Kumar K.V
2020-02-14 20:59       ` Jeff Moyer [this message]
2020-02-14 23:05         ` Dan Williams
2020-02-13  0:48 ` [PATCH v2 2/4] libnvdimm/namespace: Enforce memremap_compat_align() Dan Williams
2020-02-13 19:16   ` Jeff Moyer
2020-02-13 21:55   ` Jeff Moyer
2020-02-13 22:43     ` Dan Williams
2020-02-14 16:44       ` Jeff Moyer
2020-02-14 16:55         ` Aneesh Kumar K.V
2020-02-13  0:48 ` [PATCH v2 3/4] libnvdimm/region: Introduce NDD_LABELING Dan Williams
2020-02-13 19:12   ` Jeff Moyer
2020-02-13  0:48 ` [PATCH v2 4/4] libnvdimm/region: Introduce an 'align' attribute Dan Williams
2020-02-14 20:19   ` Jeff Moyer
2020-02-14 21:03 ` [PATCH v2 0/4] libnvdimm: Cross-arch compatible namespace alignment Jeff Moyer

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