linux-kernel - Re: [PATCH v2] x86/mm: avoid premature success when changing page attributes

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Message-ID: <20160209143005.GA3665@gmail.com>
Date:	Tue, 9 Feb 2016 15:30:05 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, mingo@...e.hu,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [PATCH v2] x86/mm: avoid premature success when changing page
 attributes


* Jan Beulich <JBeulich@...e.com> wrote:

> >>> On 28.01.16 at 09:42, <mingo@...nel.org> wrote:
> > Could we try a v3?
> 
> Okay, I withdraw the patch: Upon further consideration it is note really clear 
> what the intended behavior of set_memory_*() on address ranges with mapping 
> holes is supposed to be. The original issue was with set_memory_nx() (called 
> from mark_rodata_ro()) stumbling across an unmapped region (resulting from an 
> out of tree change completely unmapping the kernel mappings of address ranges 
> passed to free_init_pages()). [...]

So it still looks like a legitimate fix to me, even though your testcase was in an 
out of tree context:

> [...] I simply don't have the time to check whether the unmapping done with 
> CONFIG_DEBUG_PAGEALLOC would have a similar effect. The net result in any event 
> were pages (past the hole) reported as problematic when CONFIG_DEBUG_WX is 
> enabled.

Adding all the above information to the changelog addresses most of my complaints 
about it. You can also rephrase the DEBUG_PAGEALLOC bit to something like:

   I'm not completely sure about whether the unmapping done with
   CONFIG_DEBUG_PAGEALLOC would have a similar effect.

as it's perfectly fine to submit fixes you couldn't fully test.

Thanks,

	Ingo