linux-kernel - Re: [patch] x86, mm: avoid stale tlb entries by clearing prev mm

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Message-ID: <AANLkTimuSV273ksidN_vQ4o1rLr6D--9Q3dNnBiB13-K@mail.gmail.com>
Date:	Thu, 3 Feb 2011 11:13:49 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>
Subject: Re: [patch] x86, mm: avoid stale tlb entries by clearing prev
 mm_cpumask after switching mm

On Thu, Feb 3, 2011 at 10:27 AM, Suresh Siddha
<suresh.b.siddha@...el.com> wrote:
>> Actually, for normal entries I could well imagine the code that wants
>> to do a flush before freeing the page caring.
>>
>> So I think the _patch_ may be correct, but the changelog is definitely
>> not correct, and needs serious surgery to explain what the bug that
>> this fixes actually is.
>
> Linus, I updated the changelog to explain the failing case in more
> detail. Please review. Thanks.

So this explains the bug, and the explanation looks good. Except for
one (large) detail: it has nothing to do with "stale" entries.

The problematic entries are simply _bogus_, not stale. They were never
valid to begin with. So the subject and the initial part is very
misleading.

The global bit seems to be largely irrelevant too, except for the fact
that a global bogus entry obviously stays around and causes way more
trouble. But I could imagine that there could be trouble with entries
that have conflicting PSE or cacheability issues (causing machine
checks or something) even if they are ephemeral and not global.

                                  Linus
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