linux-kernel - Re: [PATCH RT] x86/mm/pat: disable preemption __split_large_page() after spin

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Date:   Tue, 08 Jan 2019 18:39:15 -0600
From:   Scott Wood <swood@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>, stable-rt@...r.kernel.org
Subject: Re: [PATCH RT] x86/mm/pat: disable preemption __split_large_page()
 after spin_lock()

On Thu, 2018-12-13 at 17:44 +0100, Sebastian Andrzej Siewior wrote:
> Commit "x86/mm/pat: Disable preemption around __flush_tlb_all()" added a
> warning if __flush_tlb_all() is invoked in preemptible context. On !RT
> the warning does not trigger because a spin lock is acquired which
> disables preemption. On RT the spin lock does not disable preemption and
> so the warning is seen.
> 
> Disable preemption to avoid the warning in __flush_tlb_all().
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

FWIW, that __flush_tlb_all() can probably just be removed.  It was added as
a partial workaround for an erratum, but since it didn't completely solve
the problem, hugepages were disabled on the affected chips in commit
7a0fc404ae66377 ("x86: Disable large pages on CPUs with Atom erratum
AAE44").  I was going to send a (non-RT) patch removing the flush but in
4.20 commit c0a759abf5a68 ("x86/mm/cpa: Move flush_tlb_all()") reworked the
code to make it moot.

-Scott