linux-kernel - Re: [PATCH 1/1] x86: restore the write back cache of reserved RAM in iounmap()

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Date:   Thu, 19 Nov 2020 22:01:02 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andrea Arcangeli <aarcange@...hat.com>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>, Rafael Aquini <aquini@...hat.com>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] x86: restore the write back cache of reserved RAM in iounmap()

On Thu, Nov 19 2020 at 12:59, Andrea Arcangeli wrote:
> If reserved memory is mapped with ioremap_noncache() or ioremap_wc(),
> the kernel correctly splits the direct mapping and marks the PAGE_SIZE
> granular region uncached, so both the virtual direct mapping and the
> second virtual mapping in vmap space will be both marked uncached or
> write through (i.e.  _PAGE_PCD/PWT set on the pagetable).
>
> However when iounmap is called later, nothing restores the direct
> mapping write back memtype.

Darn. This was discussed in 2008 already and survived that long?

  https://lore.kernel.org/lkml/20080205011357.GA14712@linux-os.sc.intel.com/

No idea how that happened to slip through...

Thanks,

        tglx