netdev - Re: [PATCH 02/19] mm/gup: factor out duplicate code from four routines

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Date:   Thu, 31 Oct 2019 11:43:37 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Ira Weiny <ira.weiny@...el.com>
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>, Jan Kara <jack@...e.cz>,
        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 02/19] mm/gup: factor out duplicate code from four
 routines

On 10/31/19 11:35 AM, Ira Weiny wrote:
> On Wed, Oct 30, 2019 at 03:49:13PM -0700, John Hubbard wrote:
...
>> +
>> +static void __remove_refs_from_head(struct page *page, int refs)
>> +{
>> +	/* Do a get_page() first, in case refs == page->_refcount */
>> +	get_page(page);
>> +	page_ref_sub(page, refs);
>> +	put_page(page);
>> +}
> 
> I wonder if this is better implemented as "put_compound_head()"?  To match the
> try_get_compound_head() call below?

Hi Ira,

Good idea, I'll rename it to that.

> 
>> +
>> +static int __huge_pt_done(struct page *head, int nr_recorded_pages, int *nr)
>> +{
>> +	*nr += nr_recorded_pages;
>> +	SetPageReferenced(head);
>> +	return 1;
> 
> When will this return anything but 1?
> 

Never, but it saves a line at all four call sites, by having it return like that.

I could see how maybe people would prefer to just have it be a void function,
and return 1 directly at the call sites. Since this was a lower line count I
thought maybe it would be slightly better, but it's hard to say really.

thanks,

John Hubbard
NVIDIA