linux-kernel - Re: [PATCH v5 02/24] mm/gup: factor out duplicate code from four routines

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Date:   Mon, 18 Nov 2019 23:00:33 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <bpf@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <kvm@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <netdev@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>
Subject: Re: [PATCH v5 02/24] mm/gup: factor out duplicate code from four
 routines

On 11/18/19 1:46 AM, Jan Kara wrote:
> On Thu 14-11-19 21:53:18, John Hubbard wrote:
>> There are four locations in gup.c that have a fair amount of code
>> duplication. This means that changing one requires making the same
>> changes in four places, not to mention reading the same code four
>> times, and wondering if there are subtle differences.
>>
>> Factor out the common code into static functions, thus reducing the
>> overall line count and the code's complexity.
>>
>> Also, take the opportunity to slightly improve the efficiency of the
>> error cases, by doing a mass subtraction of the refcount, surrounded
>> by get_page()/put_page().
>>
>> Also, further simplify (slightly), by waiting until the the successful
>> end of each routine, to increment *nr.
>>
>> Reviewed-by: Jérôme Glisse <jglisse@...hat.com>
>> Cc: Jan Kara <jack@...e.cz>
>> Cc: Ira Weiny <ira.weiny@...el.com>
>> Cc: Christoph Hellwig <hch@....de>
>> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
>> Signed-off-by: John Hubbard <jhubbard@...dia.com>
>> ---
>>  mm/gup.c | 95 ++++++++++++++++++++++++--------------------------------
>>  1 file changed, 40 insertions(+), 55 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 85caf76b3012..858541ea30ce 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -1969,6 +1969,29 @@ static int __gup_device_huge_pud(pud_t pud, pud_t *pudp, unsigned long addr,
>>  }
>>  #endif
>>  
>> +static int __record_subpages(struct page *page, unsigned long addr,
>> +			     unsigned long end, struct page **pages)
>> +{
>> +	int nr = 0;
>> +	int nr_recorded_pages = 0;
>> +
>> +	do {
>> +		pages[nr] = page;
>> +		nr++;
>> +		page++;
>> +		nr_recorded_pages++;
>> +	} while (addr += PAGE_SIZE, addr != end);
>> +	return nr_recorded_pages;
> 
> nr == nr_recorded_pages so no need for both... BTW, structuring this as a
> for loop would be probably more logical and shorter now:
> 
> 	for (nr = 0; addr != end; addr += PAGE_SIZE)
> 		pages[nr++] = page++;
> 	return nr;
> 

Nice touch, I've applied it.

thanks,
-- 
John Hubbard
NVIDIA



> The rest of the patch looks good to me.
> 
> 								Honza
>