linux-kernel - Re: [PATCH 4/5] mm: fix cache mode of dax pmd mappings

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Message-ID: <CAPcyv4gZK33SH7FjZDYpkf-9peeZ0E3pZu=ZjmHwSBxdh4zj-g@mail.gmail.com>
Date:   Tue, 6 Sep 2016 10:32:16 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <mawilcox@...rosoft.com>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Toshi Kani <toshi.kani@....com>,
        Nilesh Choudhury <nilesh.choudhury@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kai Zhang <kai.ka.zhang@...cle.com>
Subject: Re: [PATCH 4/5] mm: fix cache mode of dax pmd mappings

On Tue, Sep 6, 2016 at 10:20 AM, Matthew Wilcox <mawilcox@...rosoft.com> wrote:
> I have no objection to this patch going in for now.
>
> Longer term, surely we want to track what mode the PFNs are mapped in?  There are various bizarre suppositions out there about how persistent memory should be mapped, and it's probably better if the kernel ignores what the user specifies and keeps everything sane.  I've read the dire warnings in the Intel architecture manual and I have no desire to deal with the inevitable bug reports on some hardware I don't own and requires twenty weeks of operation in order to observe the bug.

Is there a way for userspace to establish mappings with different
cache modes, besides via /dev/mem?  That was the motivation for
CONFIG_IO_STRICT_DEVMEM.