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

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Date:   Thu, 31 Oct 2019 14:09:55 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     John Hubbard <jhubbard@...dia.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 Thu, Oct 31, 2019 at 11:43:37AM -0700, John Hubbard wrote:
> 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.

It is a NIT perhaps but I feel like the signature of a function should stand on
it's own.  What this does is mix the meaning of this function with those
calling it.  Which IMO is not good style.

We can see what others say.

Ira

> 
> thanks,
> 
> John Hubbard
> NVIDIA
>