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Shutemov" To: John Hubbard Cc: Andrew Morton , Al Viro , Alex Williamson , Benjamin Herrenschmidt , =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= , Christoph Hellwig , Dan Williams , Daniel Vetter , Dave Chinner , David Airlie , "David S . Miller" , Ira Weiny , Jan Kara , Jason Gunthorpe , Jens Axboe , Jonathan Corbet , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Magnus Karlsson , Mauro Carvalho Chehab , Michael Ellerman , Michal Hocko , Mike Kravetz , Paul Mackerras , Shuah Khan , Vlastimil Babka , bpf@vger.kernel.org, dri-devel@lists.freedesktop.org, kvm@vger.kernel.org, linux-block@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-media@vger.kernel.org, linux-rdma@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, linux-mm@kvack.org, LKML , Christoph Hellwig , "Aneesh Kumar K . V" Subject: Re: [PATCH v11 01/25] mm/gup: factor out duplicate code from four routines Message-ID: <20191218224507.nayxmx7vvsjvyzsc@box> References: <20191216222537.491123-1-jhubbard@nvidia.com> <20191216222537.491123-2-jhubbard@nvidia.com> <20191218155211.emcegdp5uqgorfwe@box> <5719efc4-e560-b3d9-8d1f-3ae289bed289@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5719efc4-e560-b3d9-8d1f-3ae289bed289@nvidia.com> User-Agent: NeoMutt/20180716 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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