linux-kernel - Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to

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Message-ID: <CAPcyv4jTUn2gSPSB1r7p9A7VNxBf54Aa5dnGbGsomDqmbvsHLQ@mail.gmail.com>
Date:   Tue, 1 May 2018 20:02:58 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Tony Luck <tony.luck@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter()

On Tue, May 1, 2018 at 7:53 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, May 1, 2018 at 7:26 PM Dan Williams <dan.j.williams@...el.com>
> wrote:
>
>> Right, but the only way to make MCE non-fatal is to teach the machine
>> check handler about recoverable conditions. This patch teaches the
>> machine check handler how to recover copy_to_iter() errors.
>
> Why not just unmap the page and remap a new page  in its place? Meaning
> that it needs absolutely no special error handling in the callers.
>
> IOW, treat it *exactly*  like the whole page poisoning.
>
> We _have_ the technology. Why does this code think it's such a special
> snow-flake?

Because dax. There's no page cache indirection games we can play here
to poison a page and map in another page. The mapped page is 1:1
associated with the filesystem block and physical memory address.