linux-kernel - Re: [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages

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Date:   Wed, 21 Jun 2017 10:47:40 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Yazen Ghannam <yazen.ghannam@....com>
Subject: Re: [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings
 of poison pages

On Mon, Jun 19, 2017 at 08:01:47PM +0200, Borislav Petkov wrote:
> (drop stable from CC)
> 
> You could use git's --suppress-cc= option when sending.

I would if I could work out how to use it. From reading the manual
page there seem to be a few options to this, but none of them appear
to just drop a specific address (apart from my own). :-(

> > +#ifdef CONFIG_X86_64
> > +
> > +void arch_unmap_kpfn(unsigned long pfn)
> > +{
> 
> I guess you can move the ifdeffery inside the function.

If I do, then the compiler will emit an empty function. It's only
a couple of bytes for the "ret" ... but why?  I may change it
to:

   #if defined(arch_unmap_kpfn) && defined(CONFIG_MEMORY_FAILURE)

to narrow down further when we need this.

> > +#if PGDIR_SHIFT + 9 < 63 /* 9 because cpp doesn't grok ilog2(PTRS_PER_PGD) */
> 
> Please no side comments.

Ok.

> Also, explain why the build-time check. (Sign-extension going away for VA
> space yadda yadda..., 5 2/3 level paging :-))

Will add.

> Also, I'm assuming this whole "workaround" of sorts should be Intel-only?

I'd assume that other X86 implementations would face similar issues (unless
they have extremely cautious pre-fetchers and/or no speculation).

I'm also assuming that non-X86 architectures that do recovery may want this
too ... hence hooking the arch_unmap_kpfn() function into the generic
memory_failure() code.

> > +	decoy_addr = (pfn << PAGE_SHIFT) + (PAGE_OFFSET ^ BIT(63));
> > +#else
> > +#error "no unused virtual bit available"
> > +#endif
> > +
> > +	if (set_memory_np(decoy_addr, 1))
> > +		pr_warn("Could not invalidate pfn=0x%lx from 1:1 map \n", pfn);
> 
> WARNING: unnecessary whitespace before a quoted newline
> #107: FILE: arch/x86/kernel/cpu/mcheck/mce.c:1089:
> +               pr_warn("Could not invalidate pfn=0x%lx from 1:1 map \n", pfn);

Oops!  Will fix.

-Tony