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To:     Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20210408121804.10440-1-osalvador@suse.de>
 <20210408121804.10440-5-osalvador@suse.de>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v7 4/8] mm,memory_hotplug: Allocate memmap from the added
 memory range
Message-ID: <54bed4d3-631f-7d30-aa2c-f8dd2f2c6804@redhat.com>
Date:   Thu, 15 Apr 2021 13:19:59 +0200
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On 08.04.21 14:18, Oscar Salvador wrote:
> Physical memory hotadd has to allocate a memmap (struct page array) for
> the newly added memory section. Currently, alloc_pages_node() is used
> for those allocations.
> 
> This has some disadvantages:
>   a) an existing memory is consumed for that purpose
>      (eg: ~2MB per 128MB memory section on x86_64)
>   b) if the whole node is movable then we have off-node struct pages
>      which has performance drawbacks.
>   c) It might be there are no PMD_ALIGNED chunks so memmap array gets
>      populated with base pages.
> 
> This can be improved when CONFIG_SPARSEMEM_VMEMMAP is enabled.
> 
> Vmemap page tables can map arbitrary memory.
> That means that we can simply use the beginning of each memory section and
> map struct pages there.
> struct pages which back the allocated space then just need to be treated
> carefully.
> 
> Implementation wise we will reuse vmem_altmap infrastructure to override
> the default allocator used by __populate_section_memmap.
> Part of the implementation also relies on memory_block structure gaining
> a new field which specifies the number of vmemmap_pages at the beginning.
> This patch also introduces the following functions:
> 
>   - vmemmap_init_space: Initializes vmemmap pages by calling move_pfn_range_to_zone(),
> 		       calls kasan_add_zero_shadow() or the vmemmap range and marks
> 		       online as many sections as vmemmap pages fully span.
>   - vmemmap_adjust_pages: Accounts/substract vmemmap_pages to node and zone
> 			 present_pages
>   - vmemmap_deinit_space: Undoes what vmemmap_init_space does.
> 

This is a bit asynchronous; and the function names are not really expressing what is being done :) I'll try to come up with better names below.

It is worth mentioning that the real "mess" is that we want offline_pages() to properly handle zone->present_pages going to 0. Therefore, we want to manually mess with the present page count.


> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
>   drivers/base/memory.c          |  64 ++++++++++++++--
>   include/linux/memory.h         |   8 +-
>   include/linux/memory_hotplug.h |  13 ++++
>   include/linux/memremap.h       |   2 +-
>   include/linux/mmzone.h         |   7 +-
>   mm/Kconfig                     |   5 ++
>   mm/memory_hotplug.c            | 162 ++++++++++++++++++++++++++++++++++++++++-
>   mm/sparse.c                    |   2 -
>   8 files changed, 247 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index f209925a5d4e..a5e536a3e9a4 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -173,16 +173,65 @@ static int memory_block_online(struct memory_block *mem)
>   {
>   	unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
>   	unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
> +	unsigned long nr_vmemmap_pages = mem->nr_vmemmap_pages;
> +	int ret;
> +
> +	/*
> +	 * Although vmemmap pages have a different lifecycle than the pages
> +	 * they describe (they remain until the memory is unplugged), doing
> +	 * its initialization and accounting at hot-{online,offline} stage
> +	 * simplifies things a lot
> +	 */

I suggest detecting the zone in here and just passing it down to online_pages().

> +	if (nr_vmemmap_pages) {
> +		ret = vmemmap_init_space(start_pfn, nr_vmemmap_pages, mem->nid,
> +					 mem->online_type);
> +		if (ret)
> +			return ret;
> +	}
>   
> -	return online_pages(start_pfn, nr_pages, mem->online_type, mem->nid);
> +	ret = online_pages(start_pfn + nr_vmemmap_pages,
> +			   nr_pages - nr_vmemmap_pages, mem->online_type,
> +			   mem->nid);
> +
> +	/*
> +	 * Undo the work if online_pages() fails.
> +	 */
> +	if (ret && nr_vmemmap_pages) {
> +		vmemmap_adjust_pages(start_pfn, -nr_vmemmap_pages);
> +		vmemmap_deinit_space(start_pfn, nr_vmemmap_pages);
> +	}
> +
> +	return ret;
>   }

My take would be doing the present page adjustment after onlining succeeded:

static int memory_block_online(struct memory_block *mem)
{
	unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
	unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
	unsigned long nr_vmemmap_pages = mem->nr_vmemmap_pages;
	struct zone *zone;
	int ret;

	zone = mhp_get_target_zone(mem->nid, mem->online_type);

	if (nr_vmemmap_pages) {
		ret = mhp_init_memmap_on_memory(start_pfn, nr_vmemmap_pages, zone);
		if (ret)
			return ret;
	}

	ret = online_pages(start_pfn + nr_vmemmap_pages, nr_pages - nr_vmemmap_pages, zone);
	if (ret) {
		if (nr_vmemmap_pages)
			mhp_deinit_memmap_on_memory(start_pfn, nr_vmemmap_pages);
		return ret;
	}

	/*
	 * Account once onlining succeeded. If the page was unpopulated,
	 * it is now already properly populated.
	 */
	if (nr_vmemmap_pages)
		adjust_present_page_count(zone, nr_vmemmap_pages);
	return 0;		
}

And the opposite:

static int memory_block_offline(struct memory_block *mem)
{
	unsigned long start_pfn = section_nr_to_pfn(mem->start_section_nr);
	unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
	unsigned long nr_vmemmap_pages = mem->nr_vmemmap_pages;
	struct zone *zone;
	int ret;

	zone = page_zone(pfn_to_page(start_pfn));


	/*
	 * Unaccount before offlining, such that unpopulated zones can
	 * properly be torn down in offline_pages().
	 */
	if (nr_vmemmap_pages)
		adjust_present_page_count(zone, -nr_vmemmap_pages);

	ret = offline_pages(start_pfn + nr_vmemmap_pages, nr_pages - nr_vmemmap_pages);
	if (ret) {
		if (nr_vmemmap_pages)
			adjust_present_page_count(zone, +nr_vmemmap_pages);
		return ret;
	}

	if (nr_vmemmap_pages)
		mhp_deinit_memmap_on_memory(start_pfn, nr_vmemmap_pages);
	return 0;		
}

Having to do the present page adjustment manually is not completely nice,
but it's easier than manually having to mess with zones becomming populated/unpopulated
outside of online_pages()/offline_pages().


-- 
Thanks,

David / dhildenb