linux-kernel - Re: [PATCH 0/7] mm: faster get user pages

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Date:   Wed, 19 Sep 2018 14:15:28 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Keith Busch <keith.busch@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 0/7] mm: faster get user pages

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?