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

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Date:   Mon, 4 May 2020 20:05:13 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
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()

> 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 out
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[1].

-Tony

[1] well there are options both globally and at the per-process level to have
the "early" notifications delivered right away.