linux-kernel - Re: [PATCH 1/4] mm/gup: add compound page list iterator

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Message-ID: <fd4b981c-cf25-dc04-7f10-549ea16bf644@oracle.com>
Date:   Thu, 4 Feb 2021 11:27:12 +0000
From:   Joao Martins <joao.m.martins@...cle.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Doug Ledford <dledford@...hat.com>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/4] mm/gup: add compound page list iterator

On 2/3/21 11:00 PM, John Hubbard wrote:
> 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@...dia.com>
>> Signed-off-by: Joao Martins <joao.m.martins@...cle.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.
> 
Hmm, pagetails is also a tiny bit redundant. Perhaps count_subpages() instead?

count_ntails is meant to be 'count number of tails' i.e. to align terminology with head +
tails which was also suggested over the other series.

>> +{
>> +	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.
> 
/me nods

> 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.

Will do.

	Joao