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Subject[PATCH v6 09/12] mm/sparsemem: Support sub-section hotplug
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Date
The libnvdimm sub-system has suffered a series of hacks and broken
workarounds for the memory-hotplug implementation's awkward
section-aligned (128MB) granularity. For example the following backtrace
is emitted when attempting arch_add_memory() with physical address
ranges that intersect 'System RAM' (RAM) with 'Persistent Memory' (PMEM)
within a given section:

WARNING: CPU: 0 PID: 558 at kernel/memremap.c:300 devm_memremap_pages+0x3b5/0x4c0
devm_memremap_pages attempted on mixed region [mem 0x200000000-0x2fbffffff flags 0x200]
[..]
Call Trace:
dump_stack+0x86/0xc3
__warn+0xcb/0xf0
warn_slowpath_fmt+0x5f/0x80
devm_memremap_pages+0x3b5/0x4c0
__wrap_devm_memremap_pages+0x58/0x70 [nfit_test_iomap]
pmem_attach_disk+0x19a/0x440 [nd_pmem]

Recently it was discovered that the problem goes beyond RAM vs PMEM
collisions as some platform produce PMEM vs PMEM collisions within a
given section. The libnvdimm workaround for that case revealed that the
libnvdimm section-alignment-padding implementation has been broken for a
long while. A fix for that long-standing breakage introduces as many
problems as it solves as it would require a backward-incompatible change
to the namespace metadata interpretation. Instead of that dubious route
[1], address the root problem in the memory-hotplug implementation.

[1]: https://lore.kernel.org/r/155000671719.348031.2347363160141119237.stgit@dwillia2-desk3.amr.corp.intel.com
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
mm/sparse.c | 224 ++++++++++++++++++++++++++++++++++++++++-------------------
1 file changed, 150 insertions(+), 74 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index bd45bff78ca1..3411321998b1 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -83,8 +83,15 @@ static int __meminit sparse_index_init(unsigned long section_nr, int nid)
unsigned long root = SECTION_NR_TO_ROOT(section_nr);
struct mem_section *section;

+ /*
+ * An existing section is possible in the sub-section hotplug
+ * case. First hot-add instantiates, follow-on hot-add reuses
+ * the existing section.
+ *
+ * The mem_hotplug_lock resolves the apparent race below.
+ */
if (mem_section[root])
- return -EEXIST;
+ return 0;

section = sparse_index_alloc(nid);
if (!section)
@@ -338,6 +345,15 @@ static void __meminit sparse_init_one_section(struct mem_section *ms,
unsigned long pnum, struct page *mem_map,
struct mem_section_usage *usage)
{
+ /*
+ * Given that SPARSEMEM_VMEMMAP=y supports sub-section hotplug,
+ * ->section_mem_map can not be guaranteed to point to a full
+ * section's worth of memory. The field is only valid / used
+ * in the SPARSEMEM_VMEMMAP=n case.
+ */
+ if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
+ mem_map = NULL;
+
ms->section_mem_map &= ~SECTION_MAP_MASK;
ms->section_mem_map |= sparse_encode_mem_map(mem_map, pnum) |
SECTION_HAS_MEM_MAP;
@@ -743,10 +759,130 @@ static void free_map_bootmem(struct page *memmap)
#endif /* CONFIG_MEMORY_HOTREMOVE */
#endif /* CONFIG_SPARSEMEM_VMEMMAP */

+#ifndef CONFIG_MEMORY_HOTREMOVE
+static void free_map_bootmem(struct page *memmap)
+{
+}
+#endif
+
+static bool is_early_section(struct mem_section *ms)
+{
+ struct page *usage_page;
+
+ usage_page = virt_to_page(ms->usage);
+ if (PageSlab(usage_page) || PageCompound(usage_page))
+ return false;
+ else
+ return true;
+}
+
+static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
+ int nid, struct vmem_altmap *altmap)
+{
+ unsigned long mask = section_active_mask(pfn, nr_pages);
+ struct mem_section *ms = __pfn_to_section(pfn);
+ bool early_section = is_early_section(ms);
+ struct page *memmap = NULL;
+
+ if (WARN(!ms->usage || (ms->usage->map_active & mask) != mask,
+ "section already deactivated: active: %#lx mask: %#lx\n",
+ ms->usage ? ms->usage->map_active : 0, mask))
+ return;
+
+ if (WARN(!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP)
+ && nr_pages < PAGES_PER_SECTION,
+ "partial memory section removal not supported\n"))
+ return;
+
+ /*
+ * There are 3 cases to handle across two configurations
+ * (SPARSEMEM_VMEMMAP={y,n}):
+ *
+ * 1/ deactivation of a partial hot-added section (only possible
+ * in the SPARSEMEM_VMEMMAP=y case).
+ * a/ section was present at memory init
+ * b/ section was hot-added post memory init
+ * 2/ deactivation of a complete hot-added section
+ * 3/ deactivation of a complete section from memory init
+ *
+ * For 1/, when map_active does not go to zero we will not be
+ * freeing the usage map, but still need to free the vmemmap
+ * range.
+ *
+ * For 2/ and 3/ the SPARSEMEM_VMEMMAP={y,n} cases are unified
+ */
+ ms->usage->map_active ^= mask;
+ if (ms->usage->map_active == 0) {
+ unsigned long section_nr = pfn_to_section_nr(pfn);
+
+ if (!early_section) {
+ kfree(ms->usage);
+ ms->usage = NULL;
+ }
+ memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
+ ms->section_mem_map = sparse_encode_mem_map(NULL, section_nr);
+ }
+
+ if (early_section && memmap)
+ free_map_bootmem(memmap);
+ else
+ depopulate_section_memmap(pfn, nr_pages, altmap);
+}
+
+static struct page * __meminit section_activate(int nid, unsigned long pfn,
+ unsigned long nr_pages, struct vmem_altmap *altmap)
+{
+ unsigned long mask = section_active_mask(pfn, nr_pages);
+ struct mem_section *ms = __pfn_to_section(pfn);
+ struct mem_section_usage *usage = NULL;
+ struct page *memmap;
+ int rc = 0;
+
+ if (!ms->usage) {
+ usage = kzalloc(mem_section_usage_size(), GFP_KERNEL);
+ if (!usage)
+ return ERR_PTR(-ENOMEM);
+ ms->usage = usage;
+ }
+
+ if (!mask)
+ rc = -EINVAL;
+ else if (mask & ms->usage->map_active)
+ rc = -EEXIST;
+ else
+ ms->usage->map_active |= mask;
+
+ if (rc) {
+ if (usage)
+ ms->usage = NULL;
+ kfree(usage);
+ return ERR_PTR(rc);
+ }
+
+ /*
+ * The early init code does not consider partially populated
+ * initial sections, it simply assumes that memory will never be
+ * referenced. If we hot-add memory into such a section then we
+ * do not need to populate the memmap and can simply reuse what
+ * is already there.
+ */
+ if (nr_pages < PAGES_PER_SECTION && is_early_section(ms))
+ return pfn_to_page(pfn);
+
+ memmap = populate_section_memmap(pfn, nr_pages, nid, altmap);
+ if (!memmap) {
+ section_deactivate(pfn, nr_pages, nid, altmap);
+ return ERR_PTR(-ENOMEM);
+ }
+
+ return memmap;
+}
+
/**
- * sparse_add_one_section - add a memory section
+ * sparse_add_section - add a memory section, or populate an existing one
* @nid: The node to add section on
* @start_pfn: start pfn of the memory range
+ * @nr_pages: number of pfns to add in the section
* @altmap: device page map
*
* This is only intended for hotplug.
@@ -760,49 +896,30 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
unsigned long nr_pages, struct vmem_altmap *altmap)
{
unsigned long section_nr = pfn_to_section_nr(start_pfn);
- struct mem_section_usage *usage;
- struct mem_section *ms;
+ struct mem_section *ms = __pfn_to_section(start_pfn);
struct page *memmap;
int ret;

- /*
- * no locking for this, because it does its own
- * plus, it does a kmalloc
- */
ret = sparse_index_init(section_nr, nid);
if (ret < 0 && ret != -EEXIST)
return ret;
- ret = 0;
- memmap = populate_section_memmap(start_pfn, PAGES_PER_SECTION, nid,
- altmap);
- if (!memmap)
- return -ENOMEM;
- usage = kzalloc(mem_section_usage_size(), GFP_KERNEL);
- if (!usage) {
- depopulate_section_memmap(start_pfn, PAGES_PER_SECTION, altmap);
- return -ENOMEM;
- }

- ms = __pfn_to_section(start_pfn);
- if (ms->section_mem_map & SECTION_MARKED_PRESENT) {
- ret = -EEXIST;
- goto out;
- }
+ memmap = section_activate(nid, start_pfn, nr_pages, altmap);
+ if (IS_ERR(memmap))
+ return PTR_ERR(memmap);
+ ret = 0;

/*
* Poison uninitialized struct pages in order to catch invalid flags
* combinations.
*/
- page_init_poison(memmap, sizeof(struct page) * PAGES_PER_SECTION);
+ page_init_poison(pfn_to_page(start_pfn), sizeof(struct page) * nr_pages);

section_mark_present(ms);
- sparse_init_one_section(ms, section_nr, memmap, usage);
+ sparse_init_one_section(ms, section_nr, memmap, ms->usage);

-out:
- if (ret < 0) {
- kfree(usage);
- depopulate_section_memmap(start_pfn, PAGES_PER_SECTION, altmap);
- }
+ if (ret < 0)
+ section_deactivate(start_pfn, nr_pages, nid, altmap);
return ret;
}

@@ -837,54 +954,13 @@ static inline void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
}
#endif

-static void free_section_usage(struct page *memmap,
- struct mem_section_usage *usage, unsigned long pfn,
- unsigned long nr_pages, struct vmem_altmap *altmap)
-{
- struct page *usage_page;
-
- if (!usage)
- return;
-
- usage_page = virt_to_page(usage);
- /*
- * Check to see if allocation came from hot-plug-add
- */
- if (PageSlab(usage_page) || PageCompound(usage_page)) {
- kfree(usage);
- if (memmap)
- depopulate_section_memmap(pfn, nr_pages, altmap);
- return;
- }
-
- /*
- * The usemap came from bootmem. This is packed with other usemaps
- * on the section which has pgdat at boot time. Just keep it as is now.
- */
-
- if (memmap)
- free_map_bootmem(memmap);
-}
-
void sparse_remove_section(struct zone *zone, struct mem_section *ms,
unsigned long pfn, unsigned long nr_pages,
unsigned long map_offset, struct vmem_altmap *altmap)
{
- struct page *memmap = NULL;
- struct mem_section_usage *usage = NULL;
-
- if (ms->section_mem_map) {
- usage = ms->usage;
- memmap = sparse_decode_mem_map(ms->section_mem_map,
- __section_nr(ms));
- ms->section_mem_map = 0;
- ms->usage = NULL;
- }
-
- clear_hwpoisoned_pages(memmap + map_offset,
- PAGES_PER_SECTION - map_offset);
- free_section_usage(memmap, usage, section_nr_to_pfn(__section_nr(ms)),
- PAGES_PER_SECTION, altmap);
+ clear_hwpoisoned_pages(pfn_to_page(pfn) + map_offset,
+ nr_pages - map_offset);
+ section_deactivate(pfn, nr_pages, zone_to_nid(zone), altmap);
}
#endif /* CONFIG_MEMORY_HOTREMOVE */
#endif /* CONFIG_MEMORY_HOTPLUG */
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