Re: [PATCH v9 02/12] mm/sparsemem: Add helpers track active portions of a section at boot

[Wei Yang]

On Wed, Jun 05, 2019 at 02:57:59PM -0700, Dan Williams wrote:
>Prepare for hot{plug,remove} of sub-ranges of a section by tracking a
>sub-section active bitmask, each bit representing a PMD_SIZE span of the
>architecture's memory hotplug section size.
>
>The implications of a partially populated section is that pfn_valid()
>needs to go beyond a valid_section() check and read the sub-section
>active ranges from the bitmask. The expectation is that the bitmask
>(subsection_map) fits in the same cacheline as the valid_section() data,
>so the incremental performance overhead to pfn_valid() should be
>negligible.
>
>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>
>Tested-by: Jane Chu <jane.chu@xxxxxxxxxx>
>Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>---
> include/linux/mmzone.h |   29 ++++++++++++++++++++++++++++-
> mm/page_alloc.c        |    4 +++-
> mm/sparse.c            |   35 +++++++++++++++++++++++++++++++++++
> 3 files changed, 66 insertions(+), 2 deletions(-)
>
>diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>index ac163f2f274f..6dd52d544857 100644
>--- a/include/linux/mmzone.h
>+++ b/include/linux/mmzone.h
>@@ -1199,6 +1199,8 @@ struct mem_section_usage {
> 	unsigned long pageblock_flags[0];
> };
> 
>+void subsection_map_init(unsigned long pfn, unsigned long nr_pages);
>+
> struct page;
> struct page_ext;
> struct mem_section {
>@@ -1336,12 +1338,36 @@ static inline struct mem_section *__pfn_to_section(unsigned long pfn)
> 
> extern int __highest_present_section_nr;
> 
>+static inline int subsection_map_index(unsigned long pfn)
>+{
>+	return (pfn & ~(PAGE_SECTION_MASK)) / PAGES_PER_SUBSECTION;
>+}
>+
>+#ifdef CONFIG_SPARSEMEM_VMEMMAP
>+static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
>+{
>+	int idx = subsection_map_index(pfn);
>+
>+	return test_bit(idx, ms->usage->subsection_map);
>+}
>+#else
>+static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
>+{
>+	return 1;
>+}
>+#endif
>+
> #ifndef CONFIG_HAVE_ARCH_PFN_VALID
> static inline int pfn_valid(unsigned long pfn)
> {
>+	struct mem_section *ms;
>+
> 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> 		return 0;
>-	return valid_section(__nr_to_section(pfn_to_section_nr(pfn)));
>+	ms = __nr_to_section(pfn_to_section_nr(pfn));
>+	if (!valid_section(ms))
>+		return 0;
>+	return pfn_section_valid(ms, pfn);
> }
> #endif
> 
>@@ -1373,6 +1399,7 @@ void sparse_init(void);
> #define sparse_init()	do {} while (0)
> #define sparse_index_init(_sec, _nid)  do {} while (0)
> #define pfn_present pfn_valid
>+#define subsection_map_init(_pfn, _nr_pages) do {} while (0)
> #endif /* CONFIG_SPARSEMEM */
> 
> /*
>diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>index c6d8224d792e..bd773efe5b82 100644
>--- a/mm/page_alloc.c
>+++ b/mm/page_alloc.c
>@@ -7292,10 +7292,12 @@ void __init free_area_init_nodes(unsigned long *max_zone_pfn)
> 
> 	/* Print out the early node map */
> 	pr_info("Early memory node ranges\n");
>-	for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid)
>+	for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid) {
> 		pr_info("  node %3d: [mem %#018Lx-%#018Lx]\n", nid,
> 			(u64)start_pfn << PAGE_SHIFT,
> 			((u64)end_pfn << PAGE_SHIFT) - 1);
>+		subsection_map_init(start_pfn, end_pfn - start_pfn);
>+	}

Just curious about why we set subsection here?

Function free_area_init_nodes() mostly handles pgdat, if I am correct. Setup
subsection here looks like touching some lower level system data structure.

> 
> 	/* Initialise every node */
> 	mminit_verify_pageflags_layout();
>diff --git a/mm/sparse.c b/mm/sparse.c
>index 71da15cc7432..0baa2e55cfdd 100644
>--- a/mm/sparse.c
>+++ b/mm/sparse.c
>@@ -210,6 +210,41 @@ static inline unsigned long first_present_section_nr(void)
> 	return next_present_section_nr(-1);
> }
> 
>+void subsection_mask_set(unsigned long *map, unsigned long pfn,
>+		unsigned long nr_pages)
>+{
>+	int idx = subsection_map_index(pfn);
>+	int end = subsection_map_index(pfn + nr_pages - 1);
>+
>+	bitmap_set(map, idx, end - idx + 1);
>+}
>+
>+void subsection_map_init(unsigned long pfn, unsigned long nr_pages)
>+{
>+	int end_sec = pfn_to_section_nr(pfn + nr_pages - 1);
>+	int i, start_sec = pfn_to_section_nr(pfn);
>+
>+	if (!nr_pages)
>+		return;
>+
>+	for (i = start_sec; i <= end_sec; i++) {
>+		struct mem_section *ms;
>+		unsigned long pfns;
>+
>+		pfns = min(nr_pages, PAGES_PER_SECTION
>+				- (pfn & ~PAGE_SECTION_MASK));
>+		ms = __nr_to_section(i);
>+		subsection_mask_set(ms->usage->subsection_map, pfn, pfns);
>+
>+		pr_debug("%s: sec: %d pfns: %ld set(%d, %d)\n", __func__, i,
>+				pfns, subsection_map_index(pfn),
>+				subsection_map_index(pfn + pfns - 1));
>+
>+		pfn += pfns;
>+		nr_pages -= pfns;
>+	}
>+}
>+
> /* Record a memory area against a node. */
> void __init memory_present(int nid, unsigned long start, unsigned long end)
> {

-- 
Wei Yang
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