On Mon, May 4, 2020 at 1:05 PM Luck, Tony <tony.luck(a)intel.com> wrote:
> When a copy function hits a bad page and the page is not yet known to
> be bad, what does it do? (I.e. the page was believed to be fine but
> the copy function gets #MC.) Does it unmap it right away? What does
> it return?
I suspect that we will only ever find a handful of situations where the
kernel can recover from memory that has gone bad that are worth fixing
(got to be some code path that touches a meaningful fraction of memory,
otherwise we get code complexity without any meaningful payoff).
I don't think we'd want different actions for the cases of "we just found
now that this page is bad" and "we got a notification an hour ago that this
page had gone bad". Currently we treat those the same for application
errors ... SIGBUS either way.
Oh, I agree that the end result should be the same. I'm thinking more
about the mechanism and the internal API. As a somewhat silly example
of why there's a difference, the first time we try to read from bad
memory, we can expect #MC (I assume, on a sensibly functioning
platform). But, once we get the #MC, I imagine that the #MC handler
will want to unmap the page to prevent a storm of additional #MC
events on the same page -- given the awful x86 #MC design, too many
all at once is fatal. So the next time we copy_mc_to_user() or
whatever from the memory, we'll get #PF instead. Or maybe that #MC
will defer the unmap?
So the point of my questions is that the overall design should be at
least somewhat settled before anyone tries to review just the copy