linux-kernel - Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy

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Date:   Sat, 2 May 2020 17:29:57 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        stable <stable@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "Tsaur, Erwin" <erwin.tsaur@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

On Fri, May 1, 2020 at 7:09 AM Luck, Tony <tony.luck@...el.com> wrote:
>
> > Now maybe copy_to_user() should *always* work this way, but I’m not convinced.
> > Certainly put_user() shouldn’t — the result wouldn’t even be well defined. And I’m
> >  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?