Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY

[David Hildenbrand <david@redhat.com>]

On 29.09.23 10:30, Mike Rapoport wrote:
> On Thu, Sep 28, 2023 at 04:33:02PM +0800, Yajun Deng wrote:
>> memmap_init_range() would init page count of all pages, but the free
>> pages count would be reset in __free_pages_core(). There are opposite
>> operations. It's unnecessary and time-consuming when it's MEMINIT_EARLY
>> context.
>>
>> Init page count in reserve_bootmem_region when in MEMINIT_EARLY context,
>> and check the page count before reset it.
>>
>> At the same time, the INIT_LIST_HEAD in reserve_bootmem_region isn't
>> need, as it already done in __init_single_page.
>>
>> The following data was tested on an x86 machine with 190GB of RAM.
>>
>> before:
>> free_low_memory_core_early()    341ms
>>
>> after:
>> free_low_memory_core_early()    285ms
>>
>> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
>> ---
>> v4: same with v2.
>> v3: same with v2.
>> v2: check page count instead of check context before reset it.
>> v1: https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@linux.dev/
>> ---
>>   mm/mm_init.c    | 18 +++++++++++++-----
>>   mm/page_alloc.c | 20 ++++++++++++--------
>>   2 files changed, 25 insertions(+), 13 deletions(-)
>>
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 9716c8a7ade9..3ab8861e1ef3 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -718,7 +718,7 @@ static void __meminit init_reserved_page(unsigned long pfn, int nid)
>>   		if (zone_spans_pfn(zone, pfn))
>>   			break;
>>   	}
>> -	__init_single_page(pfn_to_page(pfn), pfn, zid, nid, INIT_PAGE_COUNT);
>> +	__init_single_page(pfn_to_page(pfn), pfn, zid, nid, 0);
>>   }
>>   #else
>>   static inline void pgdat_set_deferred_range(pg_data_t *pgdat) {}
>> @@ -756,8 +756,8 @@ void __meminit reserve_bootmem_region(phys_addr_t start,
>>   
>>   			init_reserved_page(start_pfn, nid);
>>   
>> -			/* Avoid false-positive PageTail() */
>> -			INIT_LIST_HEAD(&page->lru);
>> +			/* Init page count for reserved region */
> 
> Please add a comment that describes _why_ we initialize the page count here.
> 
>> +			init_page_count(page);
>>   
>>   			/*
>>   			 * no need for atomic set_bit because the struct
>> @@ -888,9 +888,17 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone
>>   		}
>>   
>>   		page = pfn_to_page(pfn);
>> -		__init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT);
>> -		if (context == MEMINIT_HOTPLUG)
>> +
>> +		/* If the context is MEMINIT_EARLY, we will init page count and
>> +		 * mark page reserved in reserve_bootmem_region, the free region
>> +		 * wouldn't have page count and we will check the pages count
>> +		 * in __free_pages_core.
>> +		 */
>> +		__init_single_page(page, pfn, zone, nid, 0);
>> +		if (context == MEMINIT_HOTPLUG) {
>> +			init_page_count(page);
>>   			__SetPageReserved(page);
> 
> Rather than calling init_page_count() and __SetPageReserved() for
> MEMINIT_HOTPLUG you can set flags to INIT_PAGE_COUNT | INIT_PAGE_RESERVED
> an call __init_single_page() after the check for MEMINIT_HOTPLUG.
> 
> But more generally, I wonder if we have to differentiate HOTPLUG here at all.
> @David, can you comment please?

There are a lot of details to that, and I'll share some I can briefly think of.

1) __SetPageReserved()

I tried removing that a while ago, but there was a blocker (IIRC something about
ZONE_DEVICE). I still have the patches at [1] and I could probably take a look
if that blocker still exists (I recall that something changed at some point, but
I never had the time to follow up).

But once we stop setting the pages reserved, we might run into issues with ...


2) init_page_count()

virtio-mem, XEN balloon and HV-balloon add memory blocks that can contain holes.
set_online_page_callback() is used to intercept memory onlining and to expose
only the pages that are not holes to the buddy: calling generic_online_page() on !hole.

Holes are PageReserved but with an initialized page count. Memory offlining will fail on
PageReserved pages -- has_unmovable_pages().


At least virtio-mem clears the PageReserved flag of holes when onlining memory,
and currently relies in the page count to be reasonable (so memory offlining can work).

static void virtio_mem_set_fake_offline(unsigned long pfn,
					unsigned long nr_pages, bool onlined)
{
	page_offline_begin();
	for (; nr_pages--; pfn++) {
		struct page *page = pfn_to_page(pfn);

		__SetPageOffline(page);
		if (!onlined) {
			SetPageDirty(page);
			/* FIXME: remove after cleanups */
			ClearPageReserved(page);
		}
	}
	page_offline_end();
}


For virtio-mem, we could initialize the page count there instead. The other PV drivers
might require a bit more thought.


[1] https://github.com/davidhildenbrand/linux/tree/online_reserved_cleanup

> 
>> +		}
>>   
>>   		/*
>>   		 * Usually, we want to mark the pageblock MIGRATE_MOVABLE,
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 06be8821d833..b868caabe8dc 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1285,18 +1285,22 @@ void __free_pages_core(struct page *page, unsigned int order)
>>   	unsigned int loop;
>>   
>>   	/*
>> -	 * When initializing the memmap, __init_single_page() sets the refcount
>> -	 * of all pages to 1 ("allocated"/"not free"). We have to set the
>> -	 * refcount of all involved pages to 0.
>> +	 * When initializing the memmap, memmap_init_range sets the refcount
>> +	 * of all pages to 1 ("reserved" and "free") in hotplug context. We
>> +	 * have to set the refcount of all involved pages to 0. Otherwise,
>> +	 * we don't do it, as reserve_bootmem_region only set the refcount on
>> +	 * reserve region ("reserved") in early context.
>>   	 */
> 
> Again, why hotplug and early init should be different?
> 
>> -	prefetchw(p);
>> -	for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
>> -		prefetchw(p + 1);
>> +	if (page_count(page)) {
>> +		prefetchw(p);
>> +		for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
>> +			prefetchw(p + 1);
>> +			__ClearPageReserved(p);
>> +			set_page_count(p, 0);
>> +		}
>>   		__ClearPageReserved(p);
>>   		set_page_count(p, 0);

That looks wrong. if the page count would by pure luck be 0 already for hotplugged memory,
you wouldn't clear the reserved flag.

These changes make me a bit nervous.

-- 
Cheers,

David / dhildenb