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

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Date:   Tue, 1 May 2018 20:22:05 -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 8:20 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Tue, May 1, 2018 at 8:13 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Tue, May 1, 2018 at 8:03 PM Dan Williams <dan.j.williams@...el.com>
>> wrote:
>>
>>> 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.
>>
>> I'm not talking page cache indirection.
>>
>> I'm talking literally mapping a different page into the kernel virtual
>> address space that the failing read was done for.
>>
>> But you seem to be right that we don't actually support that. I'm guessing
>> the hwpoison code has never had to run in that kind of situation and will
>> just give up.
>>
>> That would seem to be sad. It really feels like the obvious solution to any
>> MCE's - just map a dummy page at the address that causes problems.
>>
>> That can have bad effects for real memory (because who knows what internal
>> kernel data structure might be in there), but would seem to be the
>> _optimal_ solution for some  random pmem access. And makes it absolutely
>> trivial to just return to the execution that got  the error exception.
>
> The other property of pmem that we need to contend with that makes it
> a snowflake relative to typical memory is that errors can be repaired
> by sending a slow-path command to the DIMM device. We trap block-layer
> writes in the pmem driver that target known 'badblocks' and send the
> sideband command to clear the error along with the new data.

All that to say that having a typical RAM page covering poisoned pmem
would complicate the 'clear badblocks' implementation.