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SubjectRe: [PATCH] arm64: Add memory hotplug support
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On 11/12/2018 16:36, Will Deacon wrote:
> On Mon, Dec 10, 2018 at 03:29:01PM +0000, Robin Murphy wrote:
>> Wire up the basic support for hot-adding memory. Since memory hotplug
>> is fairly tightly coupled to sparsemem, we tweak pfn_valid() to also
>> cross-check the presence of a section in the manner of the generic
>> implementation, before falling back to memblock to check for no-map
>> regions within a present section as before. By having arch_add_memory(()
>> create the linear mapping first, this then makes everything work in the
>> way that __add_section() expects.
>>
>> We expect hotplug to be ACPI-driven, so the swapper_pg_dir updates
>> should be safe from races by virtue of the global device hotplug lock.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>> ---
>>
>> Looks like I'm not going to have the whole pte_devmap story figured out
>> in time to land any ZONE_DEVICE support this cycle, but since this patch
>> also stands alone as a complete feature (and has ended up remarkably
>> simple and self-contained), I hope we might consider getting it merged
>> on its own merit.
>>
>> Robin.
>>
>> arch/arm64/Kconfig | 3 +++
>> arch/arm64/mm/init.c | 8 ++++++++
>> arch/arm64/mm/mmu.c | 12 ++++++++++++
>> arch/arm64/mm/numa.c | 10 ++++++++++
>> 4 files changed, 33 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 6d2b25f51bb3..7b855ae45747 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -261,6 +261,9 @@ config ZONE_DMA32
>> config HAVE_GENERIC_GUP
>> def_bool y
>>
>> +config ARCH_ENABLE_MEMORY_HOTPLUG
>> + def_bool y
>> +
>> config SMP
>> def_bool y
>>
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index 2983e0fc1786..82e0b08f2e31 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -291,6 +291,14 @@ int pfn_valid(unsigned long pfn)
>>
>> if ((addr >> PAGE_SHIFT) != pfn)
>> return 0;
>> +
>> +#ifdef CONFIG_SPARSEMEM
>> + if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
>> + return 0;
>> +
>> + if (!valid_section(__nr_to_section(pfn_to_section_nr(pfn))))
>> + return 0;
>
> I'm a bit nervous about the call to __nr_to_section() here. How do we
> ensure that the section number we're passing stays within the bounds of
> the mem_section array?

The same way every other sparsemem user (apart from arch/arm) does, I
guess - this is literally a copy-paste of the generic pfn_valid()
implementation :/

Given the implementation of __nr_to_section() respective of how
memory_present() and sparse_index_init() set up mem_section in the first
place, I can't see how there can be a problem. You did see the bit 4
lines above, right?

>> +#endif
>> return memblock_is_map_memory(addr);
>> }
>> EXPORT_SYMBOL(pfn_valid);
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index e1b2d58a311a..22379a74d289 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1044,3 +1044,15 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
>> pmd_free(NULL, table);
>> return 1;
>> }
>> +
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +int arch_add_memory(int nid, u64 start, u64 size, struct vmem_altmap *altmap,
>> + bool want_memblock)
>> +{
>> + __create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
>> + size, PAGE_KERNEL, pgd_pgtable_alloc, 0);
>> +
>> + return __add_pages(nid, start >> PAGE_SHIFT, size >> PAGE_SHIFT,
>> + altmap, want_memblock);
>> +}
>
> If we're mapping the new memory into the linear map, shouldn't we be
> respecting rodata_full and debug page alloc by forcing page granularity
> and tweaking the permissions?

Bah, James mentioned debug_pagealloc long ago, and I did have a slight
nagging feeling that I was still missing something - yes, I need to fix
the flags for that case. I'm not sure about rodata_full (do you mean
STRICT_KERNEL_RWX?) since a section being added here won't contain
kernel text nor data, and I can't seem to find anywhere that rodata
options affect the linear mapping of plain free RAM.

Robin.

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