linux-kernel - Re: [PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access

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Message-ID: <CA+55aFy7LvPP+DAENWftMpL=H_M5Pn9mVCHiEP7YVbFHcmbVbQ@mail.gmail.com>
Date:	Fri, 26 Oct 2012 11:41:17 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access_flags()

On Fri, Oct 26, 2012 at 11:14 AM, Rik van Riel <riel@...hat.com> wrote:
>
> I suspect the next context switch would flush out the TLB,
> making it a slowdown, not a lockup.

Common case, yes. But the page fault might happen in kernel space (due
to a "put_user()" call, say), and with CONFIG_PREEMPT=n.

Sure, put_user() is always done in a context where blocking (and
scheduling) is legal, but that doesn't necessarily equate scheduling
actually happening. If we're returning to kernel space and don't have
any IO, it might never happen.

Anyway, I suspect such behavior it's almost impossible to trigger.
Which would just make it rather hard to find.

             Linus
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