Re: [PATCH V6 3/3] arm64/mm: Enable memory hot remove

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 06/21/2019 08:05 PM, Steve Capper wrote:
> Hi Anshuman,
> 
> On Wed, Jun 19, 2019 at 09:47:40AM +0530, Anshuman Khandual wrote:
>> The arch code for hot-remove must tear down portions of the linear map and
>> vmemmap corresponding to memory being removed. In both cases the page
>> tables mapping these regions must be freed, and when sparse vmemmap is in
>> use the memory backing the vmemmap must also be freed.
>>
>> This patch adds a new remove_pagetable() helper which can be used to tear
>> down either region, and calls it from vmemmap_free() and
>> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
>> backing memory will be freed.
>>
>> remove_pagetable() makes two distinct passes over the kernel page table.
>> In the first pass it unmaps, invalidates applicable TLB cache and frees
>> backing memory if required (vmemmap) for each mapped leaf entry. In the
>> second pass it looks for empty page table sections whose page table page
>> can be unmapped, TLB invalidated and freed.
>>
>> While freeing intermediate level page table pages bail out if any of its
>> entries are still valid. This can happen for partially filled kernel page
>> table either from a previously attempted failed memory hot add or while
>> removing an address range which does not span the entire page table page
>> range.
>>
>> The vmemmap region may share levels of table with the vmalloc region.
>> There can be conflicts between hot remove freeing page table pages with
>> a concurrent vmalloc() walking the kernel page table. This conflict can
>> not just be solved by taking the init_mm ptl because of existing locking
>> scheme in vmalloc(). Hence unlike linear mapping, skip freeing page table
>> pages while tearing down vmemmap mapping.
>>
>> While here update arch_add_memory() to handle __add_pages() failures by
>> just unmapping recently added kernel linear mapping. Now enable memory hot
>> remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE.
>>
>> This implementation is overall inspired from kernel page table tear down
>> procedure on X86 architecture.
>>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
> 
> FWIW:
> Acked-by: Steve Capper <steve.capper@arm.com>

Thanks Steve.

> 
> One minor comment below though.
> 
>>  arch/arm64/Kconfig  |   3 +
>>  arch/arm64/mm/mmu.c | 290 ++++++++++++++++++++++++++++++++++++++++++++++++++--
>>  2 files changed, 284 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 6426f48..9375f26 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -270,6 +270,9 @@ config HAVE_GENERIC_GUP
>>  config ARCH_ENABLE_MEMORY_HOTPLUG
>>  	def_bool y
>>  
>> +config ARCH_ENABLE_MEMORY_HOTREMOVE
>> +	def_bool y
>> +
>>  config SMP
>>  	def_bool y
>>  
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 93ed0df..9e80a94 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -733,6 +733,250 @@ int kern_addr_valid(unsigned long addr)
>>  
>>  	return pfn_valid(pte_pfn(pte));
>>  }
>> +
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +static void free_hotplug_page_range(struct page *page, size_t size)
>> +{
>> +	WARN_ON(!page || PageReserved(page));
>> +	free_pages((unsigned long)page_address(page), get_order(size));
>> +}
> 
> We are dealing with power of 2 number of pages, it makes a lot more
> sense (to me) to replace the size parameter with order.
> 
> Also, all the callers are for known compile-time sizes, so we could just
> translate the size parameter as follows to remove any usage of get_order?
> PAGE_SIZE -> 0
> PMD_SIZE -> PMD_SHIFT - PAGE_SHIFT
> PUD_SIZE -> PUD_SHIFT - PAGE_SHIFT

Sure this can be changed but I remember Mark wanted to have this on size
instead of order which I proposed initially.