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

[Dan Williams]

On Mon, Jun 17, 2019 at 3:22 PM Wei Yang <richard.weiyang@xxxxxxxxx> wrote:
>
> 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.

Correct, I'm not sure how it ended up there, but it was the source of
a bug that was fixed with this change:

https://lore.kernel.org/lkml/CAPcyv4hjvBPDYKpp2Gns3-cc2AQ0AVS1nLk-K3fwXeRUvvzQLg@xxxxxxxxxxxxxx/