linux-kernel - Re: [PATCH] x86/e820: discard high memory that can't be addressed by 32-bit systems

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Message-ID: <aAHyHuwbmhjWmDqc@gmail.com>
Date: Fri, 18 Apr 2025 08:33:02 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Mike Rapoport <rppt@...nel.org>, Andy Shevchenko <andy@...nel.org>,
	Arnd Bergmann <arnd@...db.de>, Arnd Bergmann <arnd@...nel.org>,
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Davide Ciminaghi <ciminaghi@...dd.com>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] x86/e820: discard high memory that can't be addressed by
 32-bit systems


* Nathan Chancellor <nathan@...nel.org> wrote:

> Hi Mike,
> 
> On Sun, Apr 13, 2025 at 11:08:58AM +0300, Mike Rapoport wrote:
> ...
> >  arch/x86/kernel/e820.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> > index 57120f0749cc..5f673bd6c7d7 100644
> > --- a/arch/x86/kernel/e820.c
> > +++ b/arch/x86/kernel/e820.c
> > @@ -1300,6 +1300,14 @@ void __init e820__memblock_setup(void)
> >  		memblock_add(entry->addr, entry->size);
> >  	}
> >  
> > +	/*
> > +	 * 32-bit systems are limited to 4BG of memory even with HIGHMEM and
> > +	 * to even less without it.
> > +	 * Discard memory after max_pfn - the actual limit detected at runtime.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_X86_32))
> > +		memblock_remove(PFN_PHYS(max_pfn), -1);
> > +
> >  	/* Throw away partial pages: */
> >  	memblock_trim_memory(PAGE_SIZE);
> 
> Our CI noticed a boot failure after this change as commit 1e07b9fad022
> ("x86/e820: Discard high memory that can't be addressed by 32-bit
> systems") in -tip when booting i386_defconfig with a simple buildroot
> initrd.

I've zapped this commit from tip:x86/urgent for the time being:

  1e07b9fad022 ("x86/e820: Discard high memory that can't be addressed by 32-bit systems")

until these bugs are better understood.

Thanks,

	Ingo