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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 4 May 2020 13:26:05 -0700
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Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Williams, Dan J" <dan.j.williams@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        stable <stable@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paul Mackerras <paulus@samba.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "Tsaur, Erwin" <erwin.tsaur@intel.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, May 4, 2020 at 1:05 PM Luck, Tony <tony.luck@intel.com> wrote:
>
> > When a copy function hits a bad page and the page is not yet known to
> > be bad, what does it do?  (I.e. the page was believed to be fine but
> > the copy function gets #MC.)  Does it unmap it right away?  What does
> > it return?
>
> I suspect that we will only ever find a handful of situations where the
> kernel can recover from memory that has gone bad that are worth fixing
> (got to be some code path that touches a meaningful fraction of memory,
> otherwise we get code complexity without any meaningful payoff).
>
> I don't think we'd want different actions for the cases of "we just found out
> now that this page is bad" and "we got a notification an hour ago that this
> page had gone bad". Currently we treat those the same for application
> errors ... SIGBUS either way[1].

Oh, I agree that the end result should be the same.  I'm thinking more
about the mechanism and the internal API.  As a somewhat silly example
of why there's a difference, the first time we try to read from bad
memory, we can expect #MC (I assume, on a sensibly functioning
platform).  But, once we get the #MC, I imagine that the #MC handler
will want to unmap the page to prevent a storm of additional #MC
events on the same page -- given the awful x86 #MC design, too many
all at once is fatal.  So the next time we copy_mc_to_user() or
whatever from the memory, we'll get #PF instead.  Or maybe that #MC
will defer the unmap?

So the point of my questions is that the overall design should be at
least somewhat settled before anyone tries to review just the copy
functions.