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Subject: Re: [PATCH 2/3] mm/gup: small refactoring: simplify try_grab_page()
To:     Christoph Hellwig <hch@infradead.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>
References: <20210808235018.1924918-1-jhubbard@nvidia.com>
 <20210808235018.1924918-3-jhubbard@nvidia.com>
 <YRDNeZZQxUzbFxrQ@infradead.org>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <e46fbd0f-21f7-9cd5-d216-8ec0b9e2221d@nvidia.com>
Date:   Sun, 8 Aug 2021 23:46:08 -0700
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On 8/8/21 11:38 PM, Christoph Hellwig wrote:
> On Sun, Aug 08, 2021 at 04:50:17PM -0700, John Hubbard wrote:
>> try_grab_page() does the same thing as try_grab_compound_head(...,
>> refs=1, ...), just with a different API. So there is a lot of code
>> duplication there.
>>
>> Change try_grab_page() to call try_grab_compound_head(), while keeping
>> the API contract identical for callers.
>>
>> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
>> ---
>>   mm/gup.c | 29 ++---------------------------
>>   1 file changed, 2 insertions(+), 27 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 5cb18b62921c..4be6f060fa0b 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -203,33 +203,8 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
>>    */
>>   bool __must_check try_grab_page(struct page *page, unsigned int flags)
>>   {
>> +	if (flags & (FOLL_GET | FOLL_PIN))
>> +		return try_grab_compound_head(page, 1, flags) != NULL;
>>   
>>   	return true;
> 
> Nit: something like:
> 
> 	if (!(flags & (FOLL_GET | FOLL_PIN)))
> 		return true;
> 	return try_grab_compound_head(page, 1, flags) != NULL;
> 
> would be a little easier to read.
> 

Really? Well I'll be darned, that's what I wrote in my first draft. And then
I looked at the diffs and thought, "positive logic is clearer, and the diffs
are smaller too", and went with the current version. Which now is apparently
a little worse. oops.

Well, "50-50/90", as we used to say in an earlier job: 50% chance of either
outcome, and due to The Way Things Go, a 90% chance of picking the wrong one!

I can no longer tell which one is easier to read now, so I'll be glad to change
it. :)

thanks,
-- 
John Hubbard
NVIDIA