Re: [PATCH v7] mm/page_alloc.c: memory_hotplug: free pages as higher order

[Alexander Duyck]

On Fri, 2019-01-04 at 10:31 +0530, Arun KS wrote:
> When freeing pages are done with higher order, time spent on coalescing
> pages by buddy allocator can be reduced.  With section size of 256MB, hot
> add latency of a single section shows improvement from 50-60 ms to less
> than 1 ms, hence improving the hot add latency by 60 times.  Modify
> external providers of online callback to align with the change.
> 
> Signed-off-by: Arun KS <arunks@xxxxxxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

After running into my initial issue I actually had a few more questions
about this patch.

> [...]
> +static int online_pages_blocks(unsigned long start, unsigned long nr_pages)
> +{
> +	unsigned long end = start + nr_pages;
> +	int order, ret, onlined_pages = 0;
> +
> +	while (start < end) {
> +		order = min(MAX_ORDER - 1,
> +			get_order(PFN_PHYS(end) - PFN_PHYS(start)));
> +
> +		ret = (*online_page_callback)(pfn_to_page(start), order);
> +		if (!ret)
> +			onlined_pages += (1UL << order);
> +		else if (ret > 0)
> +			onlined_pages += ret;
> +
> +		start += (1UL << order);
> +	}
> +	return onlined_pages;
>  }
>  

Should the limit for this really be MAX_ORDER - 1 or should it be
pageblock_order? In some cases this will be the same value, but I seem
to recall that for x86 MAX_ORDER can be several times larger than
pageblock_order.

>  static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
>  			void *arg)
>  {
> -	unsigned long i;
>  	unsigned long onlined_pages = *(unsigned long *)arg;
> -	struct page *page;
>  
>  	if (PageReserved(pfn_to_page(start_pfn)))

I'm not sure we even really need this check. Getting back to the
discussion I have been having with Michal in regards to the need for
the DAX pages to not have the reserved bit cleared I was originally
wondering if we could replace this check with a call to
online_section_nr since the section shouldn't be online until we set
the bit below in online_mem_sections.

However after doing some further digging it looks like this could
probably be dropped entirely since we only call this function from
online_pages and that function is only called by memory_block_action if
pages_correctly_probed returns true. However pages_correctly_probed
should return false if any of the sections contained in the page range
is already online.

> -		for (i = 0; i < nr_pages; i++) {
> -			page = pfn_to_page(start_pfn + i);
> -			(*online_page_callback)(page);
> -			onlined_pages++;
> -		}
> +		onlined_pages = online_pages_blocks(start_pfn, nr_pages);
>  
>  	online_mem_sections(start_pfn, start_pfn + nr_pages);
>