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Date:   Thu, 17 Jan 2019 23:48:30 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "pavel@....cz" <pavel@....cz>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
        "Dock, Deneen T" <deneen.t.dock@...el.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
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        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
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        <kernel-hardening@...ts.openwall.com>,
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Subject: Re: [PATCH 14/17] mm: Make hibernate handle unmapped pages

On Fri, 2019-01-18 at 00:41 +0100, Pavel Machek wrote:
> Hi!
> 
> > > > For architectures with CONFIG_ARCH_HAS_SET_ALIAS, pages can be unmapped
> > > > briefly on the directmap, even when CONFIG_DEBUG_PAGEALLOC is not
> > > > configured.
> > > > So this changes kernel_map_pages and kernel_page_present to be defined
> > > > when
> > > > CONFIG_ARCH_HAS_SET_ALIAS is defined as well. It also changes places
> > > > (page_alloc.c) where those functions are assumed to only be implemented
> > > > when
> > > > CONFIG_DEBUG_PAGEALLOC is defined.
> > > 
> > > Which architectures are that?
> > > 
> > > Should this be merged to the patch where HAS_SET_ALIAS is introduced? We
> > > don't want broken hibernation in between....
> > 
> > Thanks for taking a look. It was added for x86 for patch 13 in this patchset
> > and
> > there was interest expressed for adding for arm64. If you didn't get the
> > whole
> > set and want to see let me know and I can send it.
> 
> I googled in in the meantime.
> 
> Anyway, if something is broken between patch 13 and 14, then they
> should be same patch.
Great. It should be ok because the new functions are not used anywhere until
after this patch.

Thanks,

Rick

> > > > -#ifdef CONFIG_DEBUG_PAGEALLOC
> > > >  extern bool _debug_pagealloc_enabled;
> > > > -extern void __kernel_map_pages(struct page *page, int numpages, int
> > > > enable);
> > > >  
> > > >  static inline bool debug_pagealloc_enabled(void)
> > > >  {
> > > > -	return _debug_pagealloc_enabled;
> > > > +	return IS_ENABLED(CONFIG_DEBUG_PAGEALLOC) &&
> > > > _debug_pagealloc_enabled;
> > > >  }
> > > 
> > > This will break build AFAICT. _debug_pagealloc_enabled variable does
> > > not exist in !CONFIG_DEBUG_PAGEALLOC case.
> > > 
> > > 									Pavel
> > 
> > After adding in the CONFIG_ARCH_HAS_SET_ALIAS condition to the ifdefs in
> > this
> > area it looked a little hard to read to me, so I moved
> > debug_pagealloc_enabled
> > and extern bool _debug_pagealloc_enabled outside to make it easier. I think
> > you
> > are right, the actual non-extern variable can not be there, but the
> > reference
> > here gets optimized out in that case.
> > 
> > Just double checked and it builds for both CONFIG_DEBUG_PAGEALLOC=n and
> > CONFIG_DEBUG_PAGEALLOC=y for me.
> 
> Ok.
> 
> Thanks,
> 									Pavel

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