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Date:   Thu, 6 Jun 2019 19:34:21 +0200
From:   Oscar Salvador <>
To:     Dan Williams <>
Cc:, Michal Hocko <>,
        Vlastimil Babka <>,
        Logan Gunthorpe <>,
        Pavel Tatashin <>,
        Benjamin Herrenschmidt <>,
        Paul Mackerras <>,
        Michael Ellerman <>,,,
Subject: Re: [PATCH v9 01/12] mm/sparsemem: Introduce struct mem_section_usage

On Wed, Jun 05, 2019 at 02:57:54PM -0700, Dan Williams wrote:
> Towards enabling memory hotplug to track partial population of a
> section, introduce 'struct mem_section_usage'.
> A pointer to a 'struct mem_section_usage' instance replaces the existing
> pointer to a 'pageblock_flags' bitmap. Effectively it adds one more
> 'unsigned long' beyond the 'pageblock_flags' (usemap) allocation to
> house a new 'subsection_map' bitmap.  The new bitmap enables the memory
> hot{plug,remove} implementation to act on incremental sub-divisions of a
> section.
> The default SUBSECTION_SHIFT is chosen to keep the 'subsection_map' no
> larger than a single 'unsigned long' on the major architectures.
> Alternatively an architecture can define ARCH_SUBSECTION_SHIFT to
> override the default PMD_SHIFT. Note that PowerPC needs to use
> ARCH_SUBSECTION_SHIFT to workaround PMD_SHIFT being a non-constant
> expression on PowerPC.
> The primary motivation for this functionality is to support platforms
> that mix "System RAM" and "Persistent Memory" within a single section,
> or multiple PMEM ranges with different mapping lifetimes within a single
> section. The section restriction for hotplug has caused an ongoing saga
> of hacks and bugs for devm_memremap_pages() users.
> Beyond the fixups to teach existing paths how to retrieve the 'usemap'
> from a section, and updates to usemap allocation path, there are no
> expected behavior changes.
> Cc: Michal Hocko <>
> Cc: Vlastimil Babka <>
> Cc: Logan Gunthorpe <>
> Cc: Oscar Salvador <>
> Cc: Pavel Tatashin <>
> Cc: Benjamin Herrenschmidt <>
> Cc: Paul Mackerras <>
> Cc: Michael Ellerman <>
> Signed-off-by: Dan Williams <>

Reviewed-by: Oscar Salvador <>

Oscar Salvador

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