linux-kernel - Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT

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Date:   Fri, 29 Sep 2023 17:50:40 +0800
From:   Yajun Deng <yajun.deng@...ux.dev>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     akpm@...ux-foundation.org, mike.kravetz@...cle.com,
        muchun.song@...ux.dev, willy@...radead.org, david@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when
 MEMINIT_EARLY


On 2023/9/29 16:30, 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@...ux.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.
Okay.
>
>> +			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.

No, the following code would cost more time than the current code in 
memmap_init().

if (context == MEMINIT_HOTPLUG)

	__init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT | INIT_PAGE_RESERVED);
else
	
	__init_single_page(page, pfn, zone, nid, 0);

> But more generally, I wonder if we have to differentiate HOTPLUG here at all.
> @David, can you comment please?
>
>> +		}
>>   
>>   		/*
>>   		 * 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?
I will add a comment that describes it will save boot time.
>
>> -	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);
>>   	}
>> -	__ClearPageReserved(p);
>> -	set_page_count(p, 0);
>>   
>>   	atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
>>   
>> -- 
>> 2.25.1
>>