linux-kernel - Re: [PATCH v2 1/4] mm/memremap_pages: Introduce memremap_compat

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Date:   Fri, 14 Feb 2020 15:05:09 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 1/4] mm/memremap_pages: Introduce memremap_compat_align()

On Fri, Feb 14, 2020 at 12:59 PM Jeff Moyer <jmoyer@...hat.com> wrote:
>
> Dan Williams <dan.j.williams@...el.com> writes:
>
> > On Thu, Feb 13, 2020 at 8:58 AM Jeff Moyer <jmoyer@...hat.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:

...but I'm not looking to change it to PAGE_SIZE, I'm going to fix the
alignment check to skip if the namespace has "inner" alignment
padding, i.e. "start_pad" and/or "end_trunc" are non-zero.