Re: [PATCH v4 04/14] mm/memremap: add ZONE_DEVICE support for compound pages

From: Christoph Hellwig <hch@lst.de>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: 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>, Christoph Hellwig <hch@lst.de>,
	nvdimm@lists.linux.dev, linux-doc@vger.kernel.org
Subject: Re: [PATCH v4 04/14] mm/memremap: add ZONE_DEVICE support for compound pages
Date: Fri, 27 Aug 2021 17:33:09 +0200	[thread overview]
Message-ID: <20210827153308.GA20687@lst.de> (raw)
In-Reply-To: <20210827145819.16471-5-joao.m.martins@oracle.com>

On Fri, Aug 27, 2021 at 03:58:09PM +0100, Joao Martins wrote:
> + * @geometry: structural definition of how the vmemmap metadata is populated.
> + *	A zero or 1 defaults to using base pages as the memmap metadata
> + *	representation. A bigger value will set up compound struct pages
> + *	representative of the requested geometry size.
>   * @ops: method table
>   * @owner: an opaque pointer identifying the entity that manages this
>   *	instance.  Used by various helpers to make sure that no
> @@ -114,6 +118,7 @@ struct dev_pagemap {
>  	struct completion done;
>  	enum memory_type type;
>  	unsigned int flags;
> +	unsigned long geometry;

So why not make this a shift as I suggested somewhere deep in the
last thread?  Also geometry sounds a bit strange, even if I can't really
offer anything better offhand.

> +static inline unsigned long pgmap_geometry(struct dev_pagemap *pgmap)
> +{
> +	if (pgmap && pgmap->geometry)
> +		return pgmap->geometry;

Why does this need to support a NULL pgmap?

> +static void __ref memmap_init_compound(struct page *head, unsigned long head_pfn,

Overly long line.