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

From: Steve Capper <[email-masked]>
Date: Fri, 21 Jun 2019 14:35:53 +0000

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 <[email-masked]>
> Signed-off-by: Anshuman Khandual <[email-masked]>
> ---

FWIW:
Acked-by: Steve Capper <[email-masked]>

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

Cheers,
-- 
Steve

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