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From: Andy Lutomirski <luto@amacapital.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Luck, Tony" <tony.luck@intel.com>,
	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>,
	Erwin Tsaur <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: Thu, 30 Apr 2020 18:20:39 -0700	[thread overview]
Message-ID: <D47C71D3-349B-49C4-9945-330C9F42A3E0@amacapital.net> (raw)
In-Reply-To: <CAHk-=wh1SPyuGkTkQESsacwKTpjWd=_-KwoCK5o=SuC3yMdf7A@mail.gmail.com>



> On Apr 30, 2020, at 5:25 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> 
> It wasn't clear how "copy_to_mc()" could ever fault. Poisoning
> after-the-fact? Why would that be preferable to just mapping a dummy
> page?

If the kernel gets an async memory error and maps a dummy page, then subsequent reads will subsequently succeed and return garbage when they should fail.  If x86 had write-only pages, we could map a dummy write-only page. But we don’t, so I think we’re stuck.

As for naming the kind of memory we’re taking about, ISTM there are two classes: DAX and monstrous memory-mapped non-persistent cache devices.  Both could be Optane, I suppose.

But I also think it’s legitimate to use these accessors to increase the chance of surviving a failure of normal memory. If a normal page happens to be page cache when it fails and if page cache access use these fancy accessors, then we might actually survive a failure.

We could be ambitious: declare that all page cache and all get_user_page’d memory should use the new accessors.  I doubt we’ll ever really succeed due to magical things like rseq and anything that thinks that users can set up their own memory as a kernel-accessed ring buffer, but I suppose we could try.
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WARNING: multiple messages have this Message-ID (diff)
From: Andy Lutomirski <luto@amacapital.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dan Williams <dan.j.williams@intel.com>,
	"Luck, Tony" <tony.luck@intel.com>,
	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>,
	Erwin Tsaur <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: Thu, 30 Apr 2020 18:20:39 -0700	[thread overview]
Message-ID: <D47C71D3-349B-49C4-9945-330C9F42A3E0@amacapital.net> (raw)
In-Reply-To: <CAHk-=wh1SPyuGkTkQESsacwKTpjWd=_-KwoCK5o=SuC3yMdf7A@mail.gmail.com>



> On Apr 30, 2020, at 5:25 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> 
> It wasn't clear how "copy_to_mc()" could ever fault. Poisoning
> after-the-fact? Why would that be preferable to just mapping a dummy
> page?

If the kernel gets an async memory error and maps a dummy page, then subsequent reads will subsequently succeed and return garbage when they should fail.  If x86 had write-only pages, we could map a dummy write-only page. But we don’t, so I think we’re stuck.

As for naming the kind of memory we’re taking about, ISTM there are two classes: DAX and monstrous memory-mapped non-persistent cache devices.  Both could be Optane, I suppose.

But I also think it’s legitimate to use these accessors to increase the chance of surviving a failure of normal memory. If a normal page happens to be page cache when it fails and if page cache access use these fancy accessors, then we might actually survive a failure.

We could be ambitious: declare that all page cache and all get_user_page’d memory should use the new accessors.  I doubt we’ll ever really succeed due to magical things like rseq and anything that thinks that users can set up their own memory as a kernel-accessed ring buffer, but I suppose we could try.


  reply	other threads:[~2020-05-01  1:20 UTC|newest]

Thread overview: 64+ 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  8:24 ` 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   ` Dan Williams
2020-05-01  2:55   ` Sasha Levin
2020-04-30  8:25 ` [PATCH v2 2/2] x86/copy_safe: Introduce copy_safe_fast() Dan Williams
2020-04-30  8:25   ` Dan Williams
2020-05-01  2:55   ` Sasha Levin
2020-04-30 14:02 ` [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Linus Torvalds
2020-04-30 14:02   ` Linus Torvalds
2020-04-30 16:51   ` Andy Lutomirski
2020-04-30 16:51     ` Andy Lutomirski
2020-04-30 17:17     ` Linus Torvalds
2020-04-30 17:17       ` Linus Torvalds
2020-04-30 18:42       ` Andy Lutomirski
2020-04-30 18:42         ` Andy Lutomirski
2020-04-30 19:22         ` Luck, Tony
2020-04-30 19:22           ` Luck, Tony
2020-04-30 19:50           ` Linus Torvalds
2020-04-30 19:50             ` Linus Torvalds
2020-04-30 20:25             ` Luck, Tony
2020-04-30 20:25               ` Luck, Tony
2020-04-30 23:52             ` Dan Williams
2020-04-30 23:52               ` Dan Williams
2020-05-01  0:10               ` Linus Torvalds
2020-05-01  0:10                 ` Linus Torvalds
2020-05-01  0:23                 ` Andy Lutomirski
2020-05-01  0:23                   ` Andy Lutomirski
2020-05-01  0:39                   ` Linus Torvalds
2020-05-01  0:39                     ` Linus Torvalds
2020-05-01  1:10                     ` Andy Lutomirski
2020-05-01  1:10                       ` Andy Lutomirski
2020-05-01 14:09                   ` Luck, Tony
2020-05-01 14:09                     ` Luck, Tony
2020-05-03  0:29                     ` Andy Lutomirski
2020-05-03  0:29                       ` Andy Lutomirski
2020-05-04 20:05                       ` Luck, Tony
2020-05-04 20:05                         ` Luck, Tony
2020-05-04 20:26                         ` Andy Lutomirski
2020-05-04 20:26                           ` Andy Lutomirski
2020-05-04 21:30                           ` Dan Williams
2020-05-04 21:30                             ` Dan Williams
2020-05-01  0:24                 ` Linus Torvalds
2020-05-01  0:24                   ` Linus Torvalds
2020-05-01  1:20                   ` Andy Lutomirski [this message]
2020-05-01  1:20                     ` Andy Lutomirski
2020-05-01  1:21                 ` Dan Williams
2020-05-01  1:21                   ` Dan Williams
2020-05-01 18:28                   ` Linus Torvalds
2020-05-01 18:28                     ` Linus Torvalds
2020-05-01 20:17                     ` Dave Hansen
2020-05-01 20:17                       ` Dave Hansen
2020-05-03 12:57                     ` David Laight
2020-05-03 12:57                       ` David Laight
2020-05-04 18:33                       ` Dan Williams
2020-05-04 18:33                         ` Dan Williams
2020-05-11 15:24                   ` Vivek Goyal
2020-05-11 15:24                     ` Vivek Goyal
2020-04-30 19:51           ` Dan Williams
2020-04-30 19:51             ` Dan Williams
2020-04-30 20:07             ` Andy Lutomirski
2020-04-30 20:07               ` Andy Lutomirski
2020-05-01  7:46         ` David Laight
2020-05-01  7:46           ` David Laight

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