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

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Message-ID: <1a77f16b-d637-40b4-bf47-a8236729a0ca@redhat.com>
Date: Tue, 19 Nov 2024 17:31:46 +0100
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.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 19.11.24 17:12, Zi Yan wrote:
> 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.

Thinking about this: why do we care about the migratetype?

We only have to fallback to pageblocks if any pageblock is 
"MIGRATE_ISOLATE" (and maybe MIGRATE_CMA), but not all. Otherwise, we 
can just ignore the migratetype (or rather overwrite it)

-- 
Cheers,

David / dhildenb