linux-kernel - Re: [PATCH mm-unstable v2 1/3] mm/contig_alloc: support __GFP

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Message-ID: <00721702-78FF-4690-8EAA-31C367FA9FC0@nvidia.com>
Date: Tue, 19 Nov 2024 11:12:06 -0500
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: Yu Zhao <yuzhao@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable v2 1/3] mm/contig_alloc: support __GFP_COMP

On 19 Nov 2024, at 10:29, David Hildenbrand wrote:

>> +/* Split a multi-block free page into its individual pageblocks. */
>> +static void split_large_buddy(struct zone *zone, struct page *page,
>> +			      unsigned long pfn, int order, fpi_t fpi)
>> +{
>> +	unsigned long end = pfn + (1 << order);
>> +
>> +	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn, 1 << order));
>> +	/* Caller removed page from freelist, buddy info cleared! */
>> +	VM_WARN_ON_ONCE(PageBuddy(page));
>> +
>> +	if (order > pageblock_order)
>> +		order = pageblock_order;
>> +
>> +	while (pfn != end) {
>> +		int mt = get_pfnblock_migratetype(page, pfn);
>> +
>> +		__free_one_page(page, pfn, zone, order, mt, fpi);
>> +		pfn += 1 << order;
>> +		page = pfn_to_page(pfn);
>> +	}
>> +}
>
> Hi,
>
> stumbling over this while digging through the code ....
>
>> +
>>   static void free_one_page(struct zone *zone, struct page *page,
>>   			  unsigned long pfn, unsigned int order,
>>   			  fpi_t fpi_flags)
>>   {
>>   	unsigned long flags;
>> -	int migratetype;
>>    	spin_lock_irqsave(&zone->lock, flags);
>> -	migratetype = get_pfnblock_migratetype(page, pfn);
>> -	__free_one_page(page, pfn, zone, order, migratetype, fpi_flags);
>
> This change is rather undesired:
>
> via __free_pages_core()->__free_pages_ok() we can easily end up here with order=MAX_PAGE_ORDER.

Do you have a concrete example? PMD THP on x86_64 is pageblock_order.
We do not have PMD level mTHP yet. Any other possible source?

>
> What your new code will do is split this perfectly reasonable MAX_PAGE_ORDER chunk via split_large_buddy() into pageblock-sized chunks, and let the buddy merging logic undo our unnecessary splitting.
>
> Is there a way to avoid this and just process the whole MAX_PAGE_ORDER chunk like we used to?

Probably split_large_buddy() can check the migratetypes of the to-be-freed
page, if order > pageblock_order. If all migratetypes are the same, the page
can be freed at MAX_PAGE_ORDER, otherwise pageblock_order.

Best Regards,
Yan, Zi