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Subject: Re: [PATCH 0/7] mm: faster get user pages
References: <20180919210250.28858-1-keith.busch@intel.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <40b392d0-0642-2d9b-5325-664a328ff677@intel.com>
Date: Wed, 19 Sep 2018 14:15:28 -0700
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To: Keith Busch <keith.busch@intel.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: Kirill Shutemov <kirill.shutemov@linux.intel.com>, Dan Williams <dan.j.williams@intel.com>

On 09/19/2018 02:02 PM, Keith Busch wrote:
> Pinning user pages out of nvdimm dax memory is significantly slower
> compared to system ram. Analysis points to software overhead incurred
> from a radix tree lookup. This patch series fixes that by removing the
> relatively costly dev_pagemap lookup that was repeated for each page,
> significantly increasing gup time.

Could you also remind us why DAX pages are such special snowflakes and
*require* radix tree lookups in the first place?