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Date: Wed, 1 Sep 2021 11:44:46 +0200
From: Christoph Hellwig <hch@lst.de>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, linux-mm@kvack.org,
	Dan Williams <dan.j.williams@intel.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	Matthew Wilcox <willy@infradead.org>,
	Jason Gunthorpe <jgg@ziepe.ca>, John Hubbard <jhubbard@nvidia.com>,
	Jane Chu <jane.chu@oracle.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jonathan Corbet <corbet@lwn.net>, nvdimm@lists.linux.dev,
	linux-doc@vger.kernel.org
Subject: Re: [PATCH v4 04/14] mm/memremap: add ZONE_DEVICE support for
 compound pages
Message-ID: <20210901094446.GA29632@lst.de>
References: <20210827145819.16471-1-joao.m.martins@oracle.com> <20210827145819.16471-5-joao.m.martins@oracle.com> <20210827153308.GA20687@lst.de> <9ee23c67-e600-555a-85fc-d527b1484bcc@oracle.com>
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On Fri, Aug 27, 2021 at 05:00:11PM +0100, Joao Martins wrote:
> So felt like doing it inline straight away inline when calling percpu_ref_get_many():
> 	
> 	(pfn_end(pgmap, range_id) - pfn_first(pgmap, range_id)) / pgmap_geometry(pgmap);
> 
> I can switch to a shift if you prefer:
> 
> 	(pfn_end(pgmap, range_id) - pfn_first(pgmap, range_id))
> 		<< pgmap_geometry_order(pgmap);

Yes.  A shift is less overhead than a branch.

> > Also geometry sounds a bit strange, even if I can't really
> > offer anything better offhand.
> > 
> We started with @align (like in device dax core), and then we switched
> to @geometry because these are slightly different things (one relates
> to vmemmap metadata structure (number of pages) and the other is how
> the mmap is aligned to a page size. I couldn't suggest anything else,
> besides a more verbose name like vmemmap_align maybe.

It for sure isn't an alignment.  I think the term that comes closest
is a granularity.  But something like vmemmap_shift if switching to
a shift might be descriptive enough for the struct member name.