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From: Joao Martins <joao.m.martins@oracle.com>
To: Christoph Hellwig <hch@lst.de>, Dan Williams <dan.j.williams@intel.com>
Cc: linux-mm@kvack.org, 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: Thu, 9 Sep 2021 10:38:52 +0100	[thread overview]
Message-ID: <f5a72a1e-7f1a-db47-4c31-512e86a749e3@oracle.com> (raw)
In-Reply-To: <20210901094446.GA29632@lst.de>

On 9/1/21 10:44 AM, Christoph Hellwig wrote:
> 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.
> 
OK.

But FWIW, the overhead of this shift or branch doesn't matter much
compared to the rest of memremap_pages().

>>> 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.
> 
Hmmm, it would be a 'shift related to vmemmap' in the literal sense but
while descriptive of the field it doesn't tell much otherwise. geometry
is probably used more widely for block device term when Dan suggested
the term. Alternatively, vmemmap_compound_nr / vmemmap_npages (if it
represents a nr of pages per compound page) or vmemmap_order (which is
more clear on what it is than vmemmap_shift?). Changing to a 'order'/'shift'
representation also gets the 'being a power of 2' enforcement for free.
And compound pages IIUC always represent a power-of-2 number of pages.

	Joao

  reply	other threads:[~2021-09-09  9:39 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
2021-09-01  9:44       ` Christoph Hellwig
2021-09-09  9:38         ` Joao Martins [this message]
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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