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

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Message-ID: <402ff3e5-4fca-4452-97ba-5b1ec4a6eeb1@redhat.com>
Date: Tue, 19 Nov 2024 16:29:06 +0100
From: David Hildenbrand <david@...hat.com>
To: Yu Zhao <yuzhao@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>, Zi Yan <ziy@...dia.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable v2 1/3] mm/contig_alloc: support __GFP_COMP

> +/* 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.

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?

-- 
Cheers,

David / dhildenb