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

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Message-ID: <AANLkTinvQatNF0ZJqmvuKYNq7oKXqfR5Ke1FuaSLFNEF@mail.gmail.com>
Date:	Wed, 2 Feb 2011 20:03:30 -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 Wed, Feb 2, 2011 at 5:55 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Feb 2, 2011 at 12:07 PM, Suresh Siddha
> <suresh.b.siddha@...el.com> wrote:
>> For the prev mm that is handing over the cpu to another mm, clear the cpu
>> from the mm_cpumask(prev) after the cr3 is changed.
>>
>> Otherwise, clearing the mm_cpumask early will avoid the flush tlb IPI's while
>> the cr3 and TLB's are still pointing to the prev mm. And this window can lead
>> to the stale (global) TLB entries.
>
> Why?
>
> This looks pointless. Explain why this matters. Global entries are
> never per-mm, so any global entries can never care about the
> mm_cpumask.
>
> And for any normal entries it doesn't matter if the IPI gets lost,
> since the TLB will be flushed (immediately afterwards) by the cr3
> write.

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
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