Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter()

[Dan Williams]

On Tue, May 1, 2018 at 8:13 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, May 1, 2018 at 8:03 PM Dan Williams <dan.j.williams@xxxxxxxxx>
> 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.