linux-kernel - Re: [LKML] Re: [PATCH] x86_64: allow sections that are recycled to set _PAGE

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Message-ID: <20100216221313.GA22869@phenom.dumpdata.com>
Date:	Tue, 16 Feb 2010 17:13:13 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"jeremy@...p.org" <jeremy@...p.org>
Subject: Re: [LKML] Re: [PATCH] x86_64: allow sections that are recycled to
	set _PAGE_RW

On Sat, Feb 13, 2010 at 12:08:17PM -0800, Suresh Siddha wrote:
> Konrad,
> 
> I don't think this patch is correct.
> 
> I am not sure if I understand why we see this failure in Xen. In native
> case, we have two kernel mappings for 64bit kernel.
> 
> One is kernel text mapping:
> 
> ffffffff80000000 - ffffffffa0000000 (=512 MB)  kernel text mapping, from
> phys 0
> 
> And another is:
> 
> ffff880000000000 - ffffc7ffffffffff (=64 TB) direct mapping of all phys.
> memory
> 
> Checks in static_protections() ensure that we map the text mapping as
> read-only (and don't bother about what permissions it uses for
> underlying free pages that get freed in free_init_pages()). But the
> kernel direct mappings for free pages will be RW and ensure that the
> free pages can be read/written using the direct mapping.
> 
> The checks in static_protections() for kernel text mapping ensure that
> we don't break the 2MB kernel text pages unnecessarily on 64bit kernels
> (as it has performance implications). We should be fine as long as the
> kernel identity mappings reflect the correct RW permissions.
> 
> But somehow this is working fine on native kernels but not on Xen pv
> guest. Your patch will cause the performance issues that we are

That would not be good.

> addressing using the static protections checks. I will look at this more
> detailed on tuesday.

Great. Thank you for doing that. If you find yourself in a bind, here are
some steps on how to build the Xen pv-ops kernel and such:
http://wiki.xensource.com/xenwiki/XenParavirtOps

It goes without saying that I would be happy to test your patch when
you have one ready.
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