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SubjectRe: [PATCH v9 08/12] mm/sparsemem: Support sub-section hotplug
On Wed, Jun 05, 2019 at 02:58:42PM -0700, Dan Williams wrote:
> 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>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> include/linux/memory_hotplug.h | 2
> mm/memory_hotplug.c | 7 -
> mm/page_alloc.c | 2
> mm/sparse.c | 225 +++++++++++++++++++++++++++-------------
> 4 files changed, 155 insertions(+), 81 deletions(-)
>
[...]
> @@ -325,6 +332,15 @@ 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;

Will this be a problem when reading mem_map with the crash-tool?
I do not expect it to be, but I am not sure if crash internally tries
to read ms->section_mem_map and do some sort of translation.
And since ms->section_mem_map SECTION_HAS_MEM_MAP, it might be that it expects
a valid mem_map?

> +static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> + struct vmem_altmap *altmap)
> +{
> + DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
> + DECLARE_BITMAP(tmp, SUBSECTIONS_PER_SECTION) = { 0 };
> + struct mem_section *ms = __pfn_to_section(pfn);
> + bool early_section = is_early_section(ms);
> + struct page *memmap = NULL;
> + unsigned long *subsection_map = ms->usage
> + ? &ms->usage->subsection_map[0] : NULL;
> +
> + subsection_mask_set(map, pfn, nr_pages);
> + if (subsection_map)
> + bitmap_and(tmp, map, subsection_map, SUBSECTIONS_PER_SECTION);
> +
> + if (WARN(!subsection_map || !bitmap_equal(tmp, map, SUBSECTIONS_PER_SECTION),
> + "section already deactivated (%#lx + %ld)\n",
> + pfn, nr_pages))
> + 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 subsection_map does not empty 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
> + */
> + bitmap_xor(subsection_map, map, subsection_map, SUBSECTIONS_PER_SECTION);
> + if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION)) {
> + 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)
> +{
> + DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
> + struct mem_section *ms = __pfn_to_section(pfn);
> + struct mem_section_usage *usage = NULL;
> + unsigned long *subsection_map;
> + struct page *memmap;
> + int rc = 0;
> +
> + subsection_mask_set(map, pfn, nr_pages);
> +
> + if (!ms->usage) {
> + usage = kzalloc(mem_section_usage_size(), GFP_KERNEL);
> + if (!usage)
> + return ERR_PTR(-ENOMEM);
> + ms->usage = usage;
> + }
> + subsection_map = &ms->usage->subsection_map[0];
> +
> + if (bitmap_empty(map, SUBSECTIONS_PER_SECTION))
> + rc = -EINVAL;
> + else if (bitmap_intersects(map, subsection_map, SUBSECTIONS_PER_SECTION))
> + rc = -EEXIST;
> + else
> + bitmap_or(subsection_map, map, subsection_map,
> + SUBSECTIONS_PER_SECTION);
> +
> + if (rc) {
> + if (usage)
> + ms->usage = NULL;
> + kfree(usage);
> + return ERR_PTR(rc);
> + }

We should not be really looking at subsection_map stuff when running on
!CONFIG_SPARSE_VMEMMAP, right?
Would it make sense to hide the bitmap dance behind

if(IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP)) ?

Sorry for nagging here

> /**
> - * 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.

Below this, the return codes are specified:

---
* Return:
* * 0 - On success.
* * -EEXIST - Section has been present.
* * -ENOMEM - Out of memory.
*/
---

We can get rid of -EEXIST since we do not return that anymore.

--
Oscar Salvador
SUSE L3

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