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Date:   Thu, 19 Nov 2020 18:02:06 +0000
From:   Christoph Hellwig <hch@infradead.org>
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Rafael Aquini <aquini@redhat.com>,
        Waiman Long <longman@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/1] x86: restore the write back cache of reserved RAM in
 iounmap()
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On Thu, Nov 19, 2020 at 12:59:01PM -0500, Andrea Arcangeli wrote:
> Hello everyone,
> 
> We identified some PCD set on the direct mapping causing hardly
> reproducible performance issues and this patch fixes the ultimate root
> cause.
> 
> The caller for now has been tweaked to avoid triggering this case (now
> that we know about it)

What is the callers?  The whole SetPageReservered + ioremap* thing
you mention in the actual patch is completely bogus.  I think we'll
need to reject that as well and fix the caller.