linux-kernel - Re: [PATCH v1 07/11] mm/huge_memory: convert split_huge_pages_pid() from follow_page() to folio

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Message-ID: <20240815100423.974775-1-p.raghav@samsung.com>
Date: Thu, 15 Aug 2024 12:04:23 +0200
From: Pankaj Raghav <p.raghav@...sung.com>
To: david@...hat.com
Cc: agordeev@...ux.ibm.com,
	akpm@...ux-foundation.org,
	borntraeger@...ux.ibm.com,
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	svens@...ux.ibm.com,
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Subject: Re: [PATCH v1 07/11] mm/huge_memory: convert split_huge_pages_pid() from follow_page() to folio_walk

Hi David,

On Fri, Aug 02, 2024 at 05:55:20PM +0200, David Hildenbrand wrote:
>  			continue;
>  		}
>  
> -		/* FOLL_DUMP to ignore special (like zero) pages */
> -		page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
> -
> -		if (IS_ERR_OR_NULL(page))
> +		folio = folio_walk_start(&fw, vma, addr, 0);
> +		if (!folio)
>  			continue;
>  
> -		folio = page_folio(page);
>  		if (!is_transparent_hugepage(folio))
>  			goto next;
>  
> @@ -3544,13 +3542,19 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
>  
>  		if (!folio_trylock(folio))
>  			goto next;
> +		folio_get(folio);

Shouldn't we lock the folio after we increase the refcount on the folio?
i.e we do folio_get() first and then folio_trylock()?

That is how it was done before (through follow_page) and this patch changes
that. Maybe it doesn't matter? To me increasing the refcount and then
locking sounds more logical but I do see this ordering getting mixed all
over the kernel.

> +		folio_walk_end(&fw, vma);
>  
>  		if (!split_folio_to_order(folio, new_order))
>  			split++;
>  
>  		folio_unlock(folio);
> -next:
>  		folio_put(folio);
> +
> +		cond_resched();
> +		continue;
> +next:
> +		folio_walk_end(&fw, vma);
>  		cond_resched();
>  	}
>  	mmap_read_unlock(mm);
> -- 
> 2.45.2

-- 
Pankaj Raghav