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Subject: Re: [PATCH v11 01/25] mm/gup: factor out duplicate code from four
 routines
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
CC:     Andrew Morton <akpm@linux-foundation.org>,
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References: <20191216222537.491123-1-jhubbard@nvidia.com>
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 <20191218155211.emcegdp5uqgorfwe@box>
From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Wed, 18 Dec 2019 14:15:53 -0800
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On 12/18/19 7:52 AM, Kirill A. Shutemov wrote:
> On Mon, Dec 16, 2019 at 02:25:13PM -0800, John Hubbard wrote:
>> +static void put_compound_head(struct page *page, int refs)
>> +{
>> +	/* Do a get_page() first, in case refs == page->_refcount */
>> +	get_page(page);
>> +	page_ref_sub(page, refs);
>> +	put_page(page);
>> +}
> 
> It's not terribly efficient. Maybe something like:
> 
> 	VM_BUG_ON_PAGE(page_ref_count(page) < ref, page);
> 	if (refs > 2)
> 		page_ref_sub(page, refs - 1);
> 	put_page(page);
> 
> ?

OK, but how about this instead? I don't see the need for a "2", as that
is a magic number that requires explanation. Whereas "1" is not a magic
number--here it means: either there are "many" (>1) refs, or not.

And the routine won't be called with refs less than about 32 (2MB huge
page, 64KB base page == 32 subpages) anyway.

	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
	/*
	 * Calling put_page() for each ref is unnecessarily slow. Only the last
	 * ref needs a put_page().
	 */
	if (refs > 1)
		page_ref_sub(page, refs - 1);
	put_page(page);


thanks,
-- 
John Hubbard
NVIDIA