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

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Date:   Wed, 19 Sep 2018 15:27:13 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Keith Busch <keith.busch@...el.com>, Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 0/7] mm: faster get user pages

On Wed, Sep 19, 2018 at 2:15 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> 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?

They are special because they need to check backing device live-ness
when taking new references. We manage a percpu-ref for each device
that registers physical memory with devm_memremap_pages(). When that
device is disabled we kill the percpu-ref to block new references
being taken, and then wait for existing references to drain. This
allows for disabling persistent-memory namepace-devices at will
relative to new get_user_pages() requests.