RE: [PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery

["Luck, Tony" <tony.luck@intel.com>]

>> But there are places in the kernel where the code assumes that this
>> EFAULT return was simply because of a page fault. The code takes some
>> action to fix that, and then retries the access. This results in a second
>> machine check.
>
> What about return EHWPOISON instead of EFAULT and update the callers
> to handle EHWPOISON explicitly: i.e., not retry but give up on the page?

That seems like a good idea to me. But I got some pushback when I started
on this path earlier with some patches to the futex code.  But back then I
wasn't using error return of EHWPOISON ... possibly the code would look
less hacky with that explicitly called out.

The futex case was specifically for code using pagefault_disable(). Likely
all the other callers would need to be audited (but there are only a few dozen
places, so not too big of a deal).

> My main concern is that the strong assumptions that the kernel can't hit more
> than a fixed number of poisoned cache lines before turning to user space
> may simply not be true.

Agreed.

> When DIMM goes bad, it can easily affect an entire bank or entire ram device
> chip. Even with memory interleaving, it's possible that a kernel control path
> touches lots of poisoned cache lines in the buffer it is working through.

These larger failures have other problems ... dozens of unrelated pages
may be affected. In a perfect world Linux would be told on the first error
that this is just one of many errors ... and be given a list. But in the real
world that isn't likely to happen :-(

-Tony