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

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Message-ID: <alpine.DEB.2.11.1601271040330.3886@nanos>
Date:	Wed, 27 Jan 2016 11:05:45 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jan Beulich <JBeulich@...e.com>
cc:	mingo@...e.hu, hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: avoid premature success when changing page
 attributes

On Mon, 25 Jan 2016, Jan Beulich wrote:

Sorry, that changelog does not make any sense.

> Since successful return from __cpa_process_fault() makes
> __change_page_attr() exit early (and successfully), its caller needs to

That has nothing to do with a successful return from __cpa_process_fault().

__change_page_attr() always returns immediately after calling
__cpa_process_fault() no matter what the return code is.

> be instructed to continue its iteration by adjusting ->numpages.

And how is that instruction working?

> While this already happens on one of __cpa_process_fault()'s successful exit
> paths, the other needs this done similarly.

Why?

> This was in particular a problem when the top level caller passed zero for
> "checkalias" (becoming the "primary" value for the other two mentioned
> functions), as is the case in change_page_attr_set_clr() when the OR of
> "mask_set" and "mask_clr" equals _PAGE_NX, as e.g. passed from
> set_memory_{,n}x().

This is completely unparseable.

Can you please describe the failure and the solution in a way, which lets one
figure out what that means w/o studying the code in detail?

Thanks,

	tglx