Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

[Andy Lutomirski]

On Fri, May 1, 2020 at 7:09 AM Luck, Tony <tony.luck@xxxxxxxxx> wrote:
>
> > Now maybe copy_to_user() should *always* work this way, but Iâm not convinced.
> > Certainly put_user() shouldnât â the result wouldnât even be well defined. And Iâm
> >  unconvinced that it makes much sense for the majority of copy_to_user() callers
> >  that are also directly accessing the source structure.
>
> One case that might work is copy_to_user() that's copying from the kernel page cache
> to the user in response to a read(2) system call.  Action would be to check if we could
> re-read from the file system to a different page. If not, return -EIO. Either way ditch the
> poison page from the page cache.
>

I think that, before we do too much design of the semantics of just
the copy function, we need a design for the whole system.
Specifically:

When the kernel finds out that a kernel page is bad (via #MC or via
any other mechanism), what does the kernel do?  Does it unmap it?
Does it replace it with a dummy page?  Does it leave it there?

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?

When a copy function hits a page that is already known to be bad
because the kernel got the "oh crap, bad page" notification earlier,
what does it do?  Return -EIO?  Take some fancier action under the
assumption that it's called in a preemptible, IRQs-on context, whereas
the original #MC or other hardware notification may have come at a
less opportune time?