linux-kernel - Re: [PATCH v4 02/23] mm/gup: factor out duplicate code from four routines

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Date:   Wed, 13 Nov 2019 15:12:02 -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 v4 02/23] mm/gup: factor out duplicate code from four
 routines

On 11/13/19 3:15 AM, Jan Kara wrote:
> On Tue 12-11-19 20:26:49, 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: 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>
> 
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 85caf76b3012..199da99e8ffc 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -1969,6 +1969,34 @@ 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)
>> +{
>> +	int nr_recorded_pages = 0;
>> +
>> +	do {
>> +		pages[nr] = page;
>> +		nr++;
>> +		page++;
>> +		nr_recorded_pages++;
>> +	} while (addr += PAGE_SIZE, addr != end);
>> +	return nr_recorded_pages;
>> +}
> 
> Why don't you pass in already pages + nr?

Aha, that does save a function argument. Will do.

...
>> +static void __huge_pt_done(struct page *head, int nr_recorded_pages, int *nr)
>> +{
>> +	*nr += nr_recorded_pages;
>> +	SetPageReferenced(head);
>> +}
> 
> I don't find this last helper very useful. It seems to muddy water more
> than necessary...

Yes, I suspect it's rather unloved, and the fact that it was hard to accurately
name should have been a big hint to not do it. I'll remove the helper and
put the lines back in directly.


thanks,
-- 
John Hubbard
NVIDIA

> 
> Other than that the cleanup looks nice to me.
> 
> 								Honza
>