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Subject: Re: [PATCH 1/4] mm/gup: add compound page list iterator
To:     Joao Martins <joao.m.martins@oracle.com>, <linux-mm@kvack.org>
CC:     <linux-kernel@vger.kernel.org>, <linux-rdma@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
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        Matthew Wilcox <willy@infradead.org>
References: <20210203220025.8568-1-joao.m.martins@oracle.com>
 <20210203220025.8568-2-joao.m.martins@oracle.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <955dbe68-7302-a8bc-f0b5-e9032d7f190e@nvidia.com>
Date:   Wed, 3 Feb 2021 15:00:01 -0800
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On 2/3/21 2:00 PM, Joao Martins wrote:
> Add an helper that iterates over head pages in a list of pages. It
> essentially counts the tails until the next page to process has a
> different head that the current. This is going to be used by
> unpin_user_pages() family of functions, to batch the head page refcount
> updates once for all passed consecutive tail pages.
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>   mm/gup.c | 29 +++++++++++++++++++++++++++++
>   1 file changed, 29 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index d68bcb482b11..4f88dcef39f2 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -215,6 +215,35 @@ void unpin_user_page(struct page *page)
>   }
>   EXPORT_SYMBOL(unpin_user_page);
>   
> +static inline unsigned int count_ntails(struct page **pages, unsigned long npages)

Silly naming nit: could we please name this function count_pagetails()? count_ntails
is a bit redundant, plus slightly less clear.

> +{
> +	struct page *head = compound_head(pages[0]);
> +	unsigned int ntails;
> +
> +	for (ntails = 1; ntails < npages; ntails++) {
> +		if (compound_head(pages[ntails]) != head)
> +			break;
> +	}
> +
> +	return ntails;
> +}
> +
> +static inline void compound_next(unsigned long i, unsigned long npages,
> +				 struct page **list, struct page **head,
> +				 unsigned int *ntails)
> +{
> +	if (i >= npages)
> +		return;
> +
> +	*ntails = count_ntails(list + i, npages - i);
> +	*head = compound_head(list[i]);
> +}
> +
> +#define for_each_compound_head(i, list, npages, head, ntails) \

When using macros, which are dangerous in general, you have to worry about
things like name collisions. I really dislike that C has forced this unsafe
pattern upon us, but of course we are stuck with it, for iterator helpers.

Given that we're stuck, you should probably use names such as __i, __list, etc,
in the the above #define. Otherwise you could stomp on existing variables.

> +	for (i = 0, compound_next(i, npages, list, &head, &ntails); \
> +	     i < npages; i += ntails, \
> +	     compound_next(i, npages, list, &head, &ntails))
> +
>   /**
>    * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
>    * @pages:  array of pages to be maybe marked dirty, and definitely released.
> 

thanks,
-- 
John Hubbard
NVIDIA