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

[Anshuman Khandual]

On 06/24/2019 10:22 PM, Mark Rutland wrote:
> On Fri, Jun 21, 2019 at 03:35:53PM +0100, 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@xxxxxxxxxx>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>> ---
>>
>> FWIW:
>> Acked-by: Steve Capper <steve.capper@xxxxxxx>
>>
>> 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
> 
> Now that I look at this again, the above makes sense to me.
> 
> I'd requested the current form (which I now realise is broken), since
> back in v2 the code looked like:
> 
> static void free_pagetable(struct page *page, int order)
> {
> 	...
> 	free_pages((unsigned long)page_address(page), order);
> 	...
> }
> 
> ... with callsites looking like:
> 
> free_pagetable(pud_page(*pud), get_order(PUD_SIZE));
> 
> ... which I now see is off by PAGE_SHIFT, and we inherited that bug in
> the current code, so the calculated order is vastly larger than it
> should be. It's worrying that doesn't seem to be caught by anything in
> testing. :/

get_order() returns the minimum page allocation order for a given size
which already takes into account PAGE_SHIFT i.e get_order(PAGE_SIZE)
returns 0.

> 
> Anshuman, could you please fold in Steve's suggested change? I'll look
> at the rest of the series shortly, so no need to resend that right away,
> but it would be worth sorting out.

get_order() is already optimized for built in constants. But will replace
with absolute constants as Steve mentioned if that is preferred.