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SubjectRe: [PATCH v2 1/4] mm/memremap_pages: Introduce memremap_compat_align()
On Fri, Feb 14, 2020 at 12:59 PM Jeff Moyer <jmoyer@redhat.com> wrote:
>
> 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:

...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.

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