linux-kernel - Re: [PATCH 1/1] mm/page_alloc: update comments in __free_pages

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Date:   Thu, 1 Dec 2022 14:41:03 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     wonder_rock@....com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/page_alloc: update comments in __free_pages_ok()

On Thu,  1 Dec 2022 21:50:45 +0800 wonder_rock@....com wrote:

> Add a comment to explain why we call get_pfnblock_migratetype() twice
> in __free_pages_ok().
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1702,6 +1702,11 @@ static void __free_pages_ok(struct page *page, unsigned int order,
>  	if (!free_pages_prepare(page, order, true, fpi_flags))
>  		return;
>  
> +	/*
> +	 * Calling get_pfnblock_migratetype() without spin_lock_irqsave() here
> +	 * is used to avoid calling get_pfnblock_migratetype() under the lock.
> +	 * This will reduce the lock holding time.
> +	 */
>  	migratetype = get_pfnblock_migratetype(page, pfn);
>  
>  	spin_lock_irqsave(&zone->lock, flags);

I guess that if the comment helped one reader, it will help others. 
But this is a pretty common trick in MM and most readers will recognize
it.

That being said, get_pfnblock_migratetype() is pretty lightweight. 
Particularly when compared with __free_one_page().  I wonder if the
optimization does much good.

If the second call to get_pfnblock_migratetype() is almost never performed
then we just added a bunch of testing and branching inside the lock
which actually made things worse!