On 2019-05-07 01:39, 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 <mhocko@xxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
arch/powerpc/include/asm/sparsemem.h | 3 +
include/linux/mmzone.h | 48 +++++++++++++++++++-
mm/memory_hotplug.c | 18 ++++----
mm/page_alloc.c | 2 -
mm/sparse.c | 81
+++++++++++++++++-----------------
5 files changed, 99 insertions(+), 53 deletions(-)
diff --git a/arch/powerpc/include/asm/sparsemem.h
b/arch/powerpc/include/asm/sparsemem.h
index 3192d454a733..1aa3c9303bf8 100644
--- a/arch/powerpc/include/asm/sparsemem.h
+++ b/arch/powerpc/include/asm/sparsemem.h
@@ -10,6 +10,9 @@
*/
#define SECTION_SIZE_BITS 24
+/* Reflect the largest possible PMD-size as the subsection-size
constant */
+#define ARCH_SUBSECTION_SHIFT 24
+
I guess this is done because PMD_SHIFT is defined at runtime rather at
compile time,
right?
#endif /* CONFIG_SPARSEMEM */
#ifdef CONFIG_MEMORY_HOTPLUG
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 70394cabaf4e..ef8d878079f9 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1160,6 +1160,44 @@ static inline unsigned long
section_nr_to_pfn(unsigned long sec)
#define SECTION_ALIGN_UP(pfn) (((pfn) + PAGES_PER_SECTION - 1) &
PAGE_SECTION_MASK)
#define SECTION_ALIGN_DOWN(pfn) ((pfn) & PAGE_SECTION_MASK)
+/*
+ * SUBSECTION_SHIFT must be constant since it is used to declare
+ * subsection_map and related bitmaps without triggering the
generation
+ * of variable-length arrays. The most natural size for a subsection
is
+ * a PMD-page. For architectures that do not have a constant PMD-size
+ * ARCH_SUBSECTION_SHIFT can be set to a constant max size, or
otherwise
+ * fallback to 2MB.
+ */
+#if defined(ARCH_SUBSECTION_SHIFT)
+#define SUBSECTION_SHIFT (ARCH_SUBSECTION_SHIFT)
+#elif defined(PMD_SHIFT)
+#define SUBSECTION_SHIFT (PMD_SHIFT)
+#else
+/*
+ * Memory hotplug enabled platforms avoid this default because they
+ * either define ARCH_SUBSECTION_SHIFT, or PMD_SHIFT is a constant,
but
+ * this is kept as a backstop to allow compilation on
+ * !ARCH_ENABLE_MEMORY_HOTPLUG archs.
+ */
+#define SUBSECTION_SHIFT 21
+#endif
+
+#define PFN_SUBSECTION_SHIFT (SUBSECTION_SHIFT - PAGE_SHIFT)
+#define PAGES_PER_SUBSECTION (1UL << PFN_SUBSECTION_SHIFT)
+#define PAGE_SUBSECTION_MASK ((~(PAGES_PER_SUBSECTION-1)))
+
+#if SUBSECTION_SHIFT > SECTION_SIZE_BITS
+#error Subsection size exceeds section size
+#else
+#define SUBSECTIONS_PER_SECTION (1UL << (SECTION_SIZE_BITS -
SUBSECTION_SHIFT))
+#endif
On powerpc, SUBSECTIONS_PER_SECTION will equal 1 (so one big section),
is that to be expected?
Will subsection_map_init handle this right?
