linux-kernel - Re: [GIT PULL] device-dax for 5.1: PMEM as RAM

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Date:   Sun, 10 Mar 2019 17:21:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux MM <linux-mm@...ck.org>,
        Dave Hansen <dave.hansen@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>
Subject: Re: [GIT PULL] device-dax for 5.1: PMEM as RAM

On Sun, Mar 10, 2019 at 4:54 PM Dan Williams <dan.j.williams@...el.com> wrote:
>
> Unfortunately this particular b0rkage is not constrained to nvmem.
> I.e. there's nothing specific about nvmem requiring mc-safe memory
> copy, it's a cpu problem consuming any poison regardless of
> source-media-type with "rep; movs".

So why is it sold and used for the nvdimm pmem driver?

People told me it was a big deal and machines died.

You can't suddenly change the story just because you want to expose it
to user space.

You can't have it both ways. Either nvdimms have more likelihood of,
and problems with, machine checks, or it doesn't.

The end result is the same: if intel believes the kernel needs to
treat nvdimms specially, then we're sure as hell not exposing those
snowflakes to user space.

And if intel *doesn't* believe that, then we're removing the mcsafe_* functions.

There's no "oh, it's safe to show to user space, but the kernel is
magical" middle ground here that makes sense to me.

                Linus