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Date: Thu, 30 Apr 2020 18:20:39 -0700 From: Andy Lutomirski <luto@...capital.net> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Dan Williams <dan.j.williams@...el.com>, "Luck, Tony" <tony.luck@...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>, Erwin Tsaur <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 Apr 30, 2020, at 5:25 PM, Linus Torvalds <torvalds@...ux-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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