linux-kernel - Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy

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Date:   Thu, 30 Apr 2020 18:20:39 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        stable <stable@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Erwin Tsaur <erwin.tsaur@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

> On Apr 30, 2020, at 5:25 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> 
> It wasn't clear how "copy_to_mc()" could ever fault. Poisoning
> after-the-fact? Why would that be preferable to just mapping a dummy
> page?

If the kernel gets an async memory error and maps a dummy page, then subsequent reads will subsequently succeed and return garbage when they should fail.  If x86 had write-only pages, we could map a dummy write-only page. But we don’t, so I think we’re stuck.

As for naming the kind of memory we’re taking about, ISTM there are two classes: DAX and monstrous memory-mapped non-persistent cache devices.  Both could be Optane, I suppose.

But I also think it’s legitimate to use these accessors to increase the chance of surviving a failure of normal memory. If a normal page happens to be page cache when it fails and if page cache access use these fancy accessors, then we might actually survive a failure.

We could be ambitious: declare that all page cache and all get_user_page’d memory should use the new accessors.  I doubt we’ll ever really succeed due to magical things like rseq and anything that thinks that users can set up their own memory as a kernel-accessed ring buffer, but I suppose we could try.