Re: [RFC PATCH 1/7] mm: sparse: set/clear subsection bitmap when pages are onlined/offlined.

["Zi Yan" <ziy@nvidia.com>]

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On 6 May 2021, at 13:48, David Hildenbrand wrote:

> On 06.05.21 17:26, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> subsection bitmap was set/cleared when a section is added/removed, but
>> pfn_to_online_page() uses subsection bitmap to check if the page is
>> online, which is not accurate. It was working when a whole section is
>> added/removed during memory hotplug and hotremove. When the following
>> patches enable memory hotplug and hotremove for subsections,
>> subsection bitmap needs to be changed during page online/offline time,
>> otherwise, pfn_to_online_page() will not give right answers. Move the
>> subsection bitmap manipulation code from section_activate() to
>> online_mem_sections() and section_deactivate() to
>> offline_mem_sections(), respectively.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   mm/sparse.c | 36 +++++++++++++++++++++++++++++++++---
>>   1 file changed, 33 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index b2ada9dc00cb..7637208b8874 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -606,6 +606,7 @@ void __init sparse_init(void)
>>    #ifdef CONFIG_MEMORY_HOTPLUG
>>  +static int fill_subsection_map(unsigned long pfn, unsigned long nr_pages);
>>   /* Mark all memory sections within the pfn range as online */
>>   void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>>   {
>> @@ -621,9 +622,12 @@ void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>>    		ms = __nr_to_section(section_nr);
>>   		ms->section_mem_map |= SECTION_IS_ONLINE;
>> +		fill_subsection_map(pfn, min(end_pfn, pfn + PAGES_PER_SECTION) - pfn);
>>   	}
>>   }
>>  +static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages);
>> +static bool is_subsection_map_empty(struct mem_section *ms);
>>   /* Mark all memory sections within the pfn range as offline */
>>   void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>>   {
>> @@ -641,7 +645,13 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>>   			continue;
>>    		ms = __nr_to_section(section_nr);
>> -		ms->section_mem_map &= ~SECTION_IS_ONLINE;
>> +
>> +		if (end_pfn < pfn + PAGES_PER_SECTION) {
>> +			clear_subsection_map(pfn, end_pfn - pfn);
>> +			if (is_subsection_map_empty(ms))
>> +				ms->section_mem_map &= ~SECTION_IS_ONLINE;
>> +		} else
>> +			ms->section_mem_map &= ~SECTION_IS_ONLINE;
>>   	}
>>   }
>>  @@ -668,6 +678,17 @@ static void free_map_bootmem(struct page *memmap)
>>   	vmemmap_free(start, end, NULL);
>>   }
>>  +static int subsection_map_intersects(struct mem_section *ms, unsigned long pfn,
>> +				     unsigned long nr_pages)
>> +{
>> +	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
>> +	unsigned long *subsection_map = &ms->usage->subsection_map[0];
>> +
>> +	subsection_mask_set(map, pfn, nr_pages);
>> +
>> +	return bitmap_intersects(map, subsection_map, SUBSECTIONS_PER_SECTION);
>> +}
>> +
>>   static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages)
>>   {
>>   	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
>> @@ -760,6 +781,12 @@ static void free_map_bootmem(struct page *memmap)
>>   	}
>>   }
>>  +static int subsection_map_intersects(struct mem_section *ms, unsigned long pfn,
>> +				     unsigned long nr_pages)
>> +{
>> +	return 0;
>> +}
>> +
>>   static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages)
>>   {
>>   	return 0;
>> @@ -800,7 +827,10 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>>   	struct page *memmap = NULL;
>>   	bool empty;
>>  -	if (clear_subsection_map(pfn, nr_pages))
>> +	if (WARN((IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) && !ms->usage) ||
>> +		 subsection_map_intersects(ms, pfn, nr_pages),
>> +				"section already deactivated (%#lx + %ld)\n",
>> +				pfn, nr_pages))
>>   		return;
>>    	empty = is_subsection_map_empty(ms);
>> @@ -855,7 +885,7 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn,
>>   		ms->usage = usage;
>>   	}
>>  -	rc = fill_subsection_map(pfn, nr_pages);
>> +	rc = !nr_pages || subsection_map_intersects(ms, pfn, nr_pages);
>>   	if (rc) {
>>   		if (usage)
>>   			ms->usage = NULL;
>>
>
> If I am not missing something, this is completely broken for devmem/ZONE_DEVICE that never onlines pages. But also when memory blocks are never onlined, this would be just wrong. Least thing you would need is a sub-section online map.

Thanks for pointing this out. I did not know that devmem/ZONE_DEVICE never onlines pages.

>
> But glimpsing at patch #2, I'd rather stop right away digging deeper into this series :)

What is the issue of patch 2, which makes pageblock_order a variable all the time? BTW, patch 2 fixes a bug by exporting pageblock_order, since when HUGETLB_PAGE_SIZE_VARIABLE is set, virtio-mem will not see pageblock_order as a variable, which could happen for PPC_BOOK2S_64 with virtio-men enabled, right? Or is this an invalid combination?

>
> I think what would really help is drafting a design of how it all could look like and then first discussing the high-level design, investigating how it could play along with all existing users, existing workloads, and existing use cases. Proposing such changes without a clear picture in mind and a high-level overview might give you some unpleasant reactions from some of the developers around here ;)

Please see my other email for a high-level design. Also I sent the patchset as a RFC to gather information on users, workloads, use cases I did not know about and I learnt a lot from your replies. :) Feedback is always welcome, but I am not sure why it needs to make people unpleasant. ;)


—
Best Regards,
Yan Zi

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