Re: [PATCH v2 0/8] mm/memory_hotplug: Factor out memory block device handling

David Hildenbrand
2019-05-07T19:21:56Z

On 07.05.19 21:04, Dan Williams wrote:
> On Tue, May 7, 2019 at 11:38 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> We only want memory block devices for memory to be onlined/offlined
>> (add/remove from the buddy). This is required so user space can
>> online/offline memory and kdump gets notified about newly onlined memory.
>>
>> Only such memory has the requirement of having to span whole memory blocks.
>> Let's factor out creation/removal of memory block devices. This helps
>> to further cleanup arch_add_memory/arch_remove_memory() and to make
>> implementation of new features easier. E.g. supplying a driver for
>> memory block devices becomes way easier (so user space is able to
>> distinguish different types of added memory to properly online it).
>>
>> Patch 1 makes sure the memory block size granularity is always respected.
>> Patch 2 implements arch_remove_memory() on s390x. Patch 3 prepares
>> arch_remove_memory() to be also called without CONFIG_MEMORY_HOTREMOVE.
>> Patch 4,5 and 6 factor out creation/removal of memory block devices.
>> Patch 7 gets rid of some unlikely errors that could have happened, not
>> removing links between memory block devices and nodes, previously brought
>> up by Oscar.
>>
>> Did a quick sanity test with DIMM plug/unplug, making sure all devices
>> and sysfs links properly get added/removed. Compile tested on s390x and
>> x86-64.
>>
>> Based on git://git.cmpxchg.org/linux-mmots.git
>>
>> Next refactoring on my list will be making sure that remove_memory()
>> will never deal with zones / access "struct pages". Any kind of zone
>> handling will have to be done when offlining system memory / before
>> removing device memory. I am thinking about remove_pfn_range_from_zone()",
>> du undo everything "move_pfn_range_to_zone()" did.
>>
>> v1 -> v2:
>> - s390x/mm: Implement arch_remove_memory()
>> -- remove mapping after "__remove_pages"
>>
>>
>> David Hildenbrand (8):
>> mm/memory_hotplug: Simplify and fix check_hotplug_memory_range()
>> s390x/mm: Implement arch_remove_memory()
>> mm/memory_hotplug: arch_remove_memory() and __remove_pages() with
>> CONFIG_MEMORY_HOTPLUG
>> mm/memory_hotplug: Create memory block devices after arch_add_memory()
>> mm/memory_hotplug: Drop MHP_MEMBLOCK_API
>
> So at a minimum we need a bit of patch staging guidance because this
> obviously collides with the subsection bits that are built on top of
> the existence of MHP_MEMBLOCK_API. What trigger do you envision as a
> replacement that arch_add_memory() use to determine that subsection
> operations should be disallowed?
>

Looks like we now have time to sort it out :)

Looking at your series

[PATCH v8 08/12] mm/sparsemem: Prepare for sub-section ranges

is the "single" effectively place using MHP_MEMBLOCK_API, namely
"subsection_check()". Used when adding/removing memory.

+static int subsection_check(unsigned long pfn, unsigned long nr_pages,
+ unsigned long flags, const char *reason)
+{
+ /*
+ * Only allow partial section hotplug for !memblock ranges,
+ * since register_new_memory() requires section alignment, and
+ * CONFIG_SPARSEMEM_VMEMMAP=n requires sections to be fully
+ * populated.
+ */
+ if ((!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP)
+ || (flags & MHP_MEMBLOCK_API))
+ && ((pfn & ~PAGE_SECTION_MASK)
+ || (nr_pages & ~PAGE_SECTION_MASK))) {
+ WARN(1, "Sub-section hot-%s incompatible with %s\n", reason,
+ (flags & MHP_MEMBLOCK_API)
+ ? "memblock api" : "!CONFIG_SPARSEMEM_VMEMMAP");
+ return -EINVAL;
+ }
+ return 0;
}

(flags & MHP_MEMBLOCK_API)) && ((pfn & ~PAGE_SECTION_MASK) || (nr_pages
& ~PAGE_SECTION_MASK)))

sounds like something the caller (add_memory()) always has to take care
of. No need to check. The one imposing this restriction is the only caller.

In my opinion, that check/function can go completely.

Am I missing something / missing another user?

--

Thanks,

David / dhildenb