linux-kernel - Re: [PATCH v11 01/25] mm/gup: factor out duplicate code from four routines

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Message-ID: <20191218224507.nayxmx7vvsjvyzsc@box>
Date:   Thu, 19 Dec 2019 01:45:07 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
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>,
        Ira Weiny <ira.weiny@...el.com>, 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 v11 01/25] mm/gup: factor out duplicate code from four
 routines

On Wed, Dec 18, 2019 at 02:15:53PM -0800, John Hubbard wrote:
> On 12/18/19 7:52 AM, Kirill A. Shutemov wrote:
> > On Mon, Dec 16, 2019 at 02:25:13PM -0800, John Hubbard wrote:
> > > +static void put_compound_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);
> > > +}
> > 
> > It's not terribly efficient. Maybe something like:
> > 
> > 	VM_BUG_ON_PAGE(page_ref_count(page) < ref, page);
> > 	if (refs > 2)
> > 		page_ref_sub(page, refs - 1);
> > 	put_page(page);
> > 
> > ?
> 
> OK, but how about this instead? I don't see the need for a "2", as that
> is a magic number that requires explanation. Whereas "1" is not a magic
> number--here it means: either there are "many" (>1) refs, or not.

Yeah, it's my thinko. Sure, it has to be '1' (or >= 2, which is less readable).

> And the routine won't be called with refs less than about 32 (2MB huge
> page, 64KB base page == 32 subpages) anyway.

It's hard to make predictions about future :P

> 	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
> 	/*
> 	 * Calling put_page() for each ref is unnecessarily slow. Only the last
> 	 * ref needs a put_page().
> 	 */
> 	if (refs > 1)
> 		page_ref_sub(page, refs - 1);
> 	put_page(page);

Looks good to me.

-- 
 Kirill A. Shutemov