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

[Wei Yang]

On Mon, Jun 17, 2019 at 03:32:45PM -0700, Dan Williams wrote:
>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/

So this one is moved to sparse_init_nid().

The bug is strange, while the code now is more reasonable to me.

Thanks :-)

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-- 
Wei Yang
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