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From: Joao Martins <joao.m.martins@oracle.com>
To: Christoph Hellwig <hch@lst.de>
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>,
	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 17:00:11 +0100	[thread overview]
Message-ID: <9ee23c67-e600-555a-85fc-d527b1484bcc@oracle.com> (raw)
In-Reply-To: <20210827153308.GA20687@lst.de>

On 8/27/21 4:33 PM, Christoph Hellwig wrote:
> 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? 

So the previous you suggested had the check for pgmap_geometry() > PAGE_SIZE,
but because pgmap_geometry() return 1 by default, then the pfn_end() - pfn
computation wouldn't change for those that don't request a geometry.

So rather than this that you initially suggested:

	refs = pfn_end(pgmap, range_id) - pfn_first(pgmap, range_id);
	if (pgmap_geometry(pgmap) > 1)
		refs /= pgmap_geometry(pgmap);

I would turn into this, because now for users which don't select geometry it's always a
division by 1:

	refs = pfn_end(pgmap, range_id) - pfn_first(pgmap, range_id);
	refs /= pgmap_geometry(pgmap);

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);

> 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.

>> +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?
> 
This was mainly a defensive choice, similar to put_dev_pagemap(). There's
only one caller, which is populate_section_memmap with an altmap but without
pgmap.

>> +static void __ref memmap_init_compound(struct page *head, unsigned long head_pfn,
> 
> Overly long line.
> 
Fixed.

Interesting that checkpatch doesn't complain with one character past 80 lines.

  reply	other threads:[~2021-08-27 16:00 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-27 14:58 [PATCH v4 00/14] mm, sparse-vmemmap: Introduce compound devmaps for device-dax Joao Martins
2021-08-27 14:58 ` [PATCH v4 01/14] memory-failure: fetch compound_head after pgmap_pfn_valid() Joao Martins
2021-08-27 14:58 ` [PATCH v4 02/14] mm/page_alloc: split prep_compound_page into head and tail subparts Joao Martins
2021-08-27 14:58 ` [PATCH v4 03/14] mm/page_alloc: refactor memmap_init_zone_device() page init Joao Martins
2021-08-27 14:58 ` [PATCH v4 04/14] mm/memremap: add ZONE_DEVICE support for compound pages Joao Martins
2021-08-27 15:33   ` Christoph Hellwig
2021-08-27 16:00     ` Joao Martins [this message]
2021-09-01  9:44       ` Christoph Hellwig
2021-09-09  9:38         ` Joao Martins
2021-08-27 14:58 ` [PATCH v4 05/14] device-dax: use ALIGN() for determining pgoff Joao Martins
2021-08-27 14:58 ` [PATCH v4 06/14] device-dax: ensure dev_dax->pgmap is valid for dynamic devices Joao Martins
2021-11-05  0:31   ` Dan Williams
2021-11-05 12:09     ` Joao Martins
2021-11-05 16:14       ` Joao Martins
2021-11-05 16:46       ` Dan Williams
2021-11-05 18:11         ` Joao Martins
2021-08-27 14:58 ` [PATCH v4 07/14] device-dax: compound devmap support Joao Martins
2021-11-05  0:38   ` Dan Williams
2021-11-05 14:10     ` Joao Martins
2021-11-05 16:41       ` Dan Williams
2021-08-27 14:58 ` [PATCH v4 08/14] mm/gup: grab head page refcount once for group of subpages Joao Martins
2021-08-27 16:25   ` Jason Gunthorpe
2021-08-27 18:34     ` Joao Martins
2021-08-30 13:07       ` Jason Gunthorpe
2021-08-31 12:34         ` Joao Martins
2021-08-31 17:05           ` Jason Gunthorpe
2021-09-23 16:51             ` Joao Martins
2021-09-28 18:01               ` Jason Gunthorpe
2021-09-29 11:50                 ` Joao Martins
2021-09-29 19:34                   ` Jason Gunthorpe
2021-09-30  3:01                     ` Alistair Popple
2021-09-30 17:54                       ` Joao Martins
2021-09-30 21:55                         ` Jason Gunthorpe
2021-10-18 18:36                       ` Jason Gunthorpe
2021-10-18 18:37                   ` Jason Gunthorpe
2021-10-08 11:54   ` Jason Gunthorpe
2021-10-11 15:53     ` Joao Martins
2021-10-13 17:41       ` Jason Gunthorpe
2021-10-13 19:18         ` Joao Martins
2021-10-13 19:43           ` Jason Gunthorpe
2021-10-14 17:56             ` Joao Martins
2021-10-14 18:06               ` Jason Gunthorpe
2021-08-27 14:58 ` [PATCH v4 09/14] mm/sparse-vmemmap: add a pgmap argument to section activation Joao Martins
2021-08-27 14:58 ` [PATCH v4 10/14] mm/sparse-vmemmap: refactor core of vmemmap_populate_basepages() to helper Joao Martins
2021-08-27 14:58 ` [PATCH v4 11/14] mm/hugetlb_vmemmap: move comment block to Documentation/vm Joao Martins
2021-08-27 14:58 ` [PATCH v4 12/14] mm/sparse-vmemmap: populate compound devmaps Joao Martins
2021-08-27 14:58 ` [PATCH v4 13/14] mm/page_alloc: reuse tail struct pages for " Joao Martins
2021-08-27 14:58 ` [PATCH v4 14/14] mm/sparse-vmemmap: improve memory savings for compound pud geometry Joao Martins

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