From: Andy Lutomirski <luto@kernel.org>
To: "Luck, Tony" <tony.luck@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Borislav Petkov <bp@alien8.de>, stable <stable@vger.kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Paul Mackerras <paulus@samba.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
"Tsaur, Erwin" <erwin.tsaur@intel.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
linux-nvdimm <linux-nvdimm@lists.01.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()
Date: Mon, 4 May 2020 13:26:05 -0700 [thread overview]
Message-ID: <CALCETrVAsppM5kRz0HicAQ8o_x06=7Nd0q64sEre3MEShWPaLw@mail.gmail.com> (raw)
In-Reply-To: <3908561D78D1C84285E8C5FCA982C28F7F612DF4@ORSMSX115.amr.corp.intel.com>
On Mon, May 4, 2020 at 1:05 PM Luck, Tony <tony.luck@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 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].
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
functions.
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next prev parent reply other threads:[~2020-05-04 20:26 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-30 8:24 [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Dan Williams
2020-04-30 8:25 ` [PATCH v2 1/2] copy_safe: Rename memcpy_mcsafe() to copy_safe() Dan Williams
2020-04-30 8:25 ` [PATCH v2 2/2] x86/copy_safe: Introduce copy_safe_fast() Dan Williams
2020-04-30 14:02 ` [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Linus Torvalds
2020-04-30 16:51 ` Andy Lutomirski
2020-04-30 17:17 ` Linus Torvalds
2020-04-30 18:42 ` Andy Lutomirski
2020-04-30 19:22 ` Luck, Tony
2020-04-30 19:50 ` Linus Torvalds
2020-04-30 20:25 ` Luck, Tony
2020-04-30 23:52 ` Dan Williams
2020-05-01 0:10 ` Linus Torvalds
2020-05-01 0:23 ` Andy Lutomirski
2020-05-01 0:39 ` Linus Torvalds
2020-05-01 1:10 ` Andy Lutomirski
2020-05-01 14:09 ` Luck, Tony
2020-05-03 0:29 ` Andy Lutomirski
2020-05-04 20:05 ` Luck, Tony
2020-05-04 20:26 ` Andy Lutomirski [this message]
2020-05-04 21:30 ` Dan Williams
2020-05-01 0:24 ` Linus Torvalds
2020-05-01 1:20 ` Andy Lutomirski
2020-05-01 1:21 ` Dan Williams
2020-05-01 18:28 ` Linus Torvalds
2020-05-01 20:17 ` Dave Hansen
2020-05-03 12:57 ` David Laight
2020-05-04 18:33 ` Dan Williams
2020-05-11 15:24 ` Vivek Goyal
2020-04-30 19:51 ` Dan Williams
2020-04-30 20:07 ` Andy Lutomirski
2020-05-01 7:46 ` David Laight
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