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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 19 Sep 2018 16:26:22 -0600
From:   Keith Busch <keith.busch@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        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 Wed, Sep 19, 2018 at 02:15:28PM -0700, Dave Hansen 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?

Yeah, ZONE_DEVICE memory is special. It has struct page mappings, but
not for online general use. The dev_pagemap is the metadata to the zone
device memory, and that metadata is stashed in a radix tree.

We're looking up the dev_pagemap in this path to take a reference so
the zone device can't be unmapped.