linux-kernel - Re: Non-linear Remap Vs. VM Cleanup: Performance Hit

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Message-ID: <20150227153905.GA12562@node.dhcp.inet.fi>
Date:	Fri, 27 Feb 2015 17:39:05 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	"D. Cooper Stevenson" <cooper@...per.stevenson.name>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Non-linear Remap Vs. VM Cleanup: Performance Hit

On Tue, Feb 24, 2015 at 02:36:25PM -0800, D. Cooper Stevenson wrote:
> Hello All,
> 
> I read the Linus's release for RC1. One of his favorite features in the
> release are, "actually some vm cleanups, where this release is getting rid
> of the largely unused non-linear remapping code (replaced with just
> emulating it with lots of smaller mappings) and unifies the NUMA and
> PROTNONE handling for page tables."
> 
> I understand that non-linear page remapping is unused (and, presumably,
> reduces the code base size) but according to the remap_file_pages man page
> (http://man7.org/linux/man-pages/man2/remap_file_pages.2.html) non-linear
> system will be, "eventually be replaced by a slower in-kernel emulation."
> 
> Was this a change of necessity? What gives?

As with all cleanups it's not necessary, but it makes life easier.

It removed >1.5k lines of non-trivial code from critical path of the
kernel. It's rarely used and tend to be error-prone. And developers need
to be aware about exists of non-linear mapping while implementing new
features.

For instance, faultaround feature I've implemented was broken for
non-linear mapping for some time and it took time to track it down.

-- 
 Kirill A. Shutemov
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