Re: [tip: x86/urgent] x86/e820: Discard high memory that can't be addressed by 32-bit systems

[Mike Rapoport]

On Mon, Apr 14, 2025 at 07:19:02AM -0700, Dave Hansen wrote:
> On 4/13/25 02:23, tip-bot2 for Mike Rapoport (Microsoft) wrote:
> > +	/*
> > +	 * 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);
> 
> Mike, thanks for the quick fix! I did verify that this gets my silly
> test VM booting again.
> 
> The patch obviously _works_. But in the case I was hitting max_pfn was
> set MAX_NONPAE_PFN. The unfortunate part about this hunk is that it's
> far away from the related warning:

Yeah, my first instinct was to put memblock_remove() in the same 'if',
but there's no memblock there yet :)
 
> >         if (max_pfn > MAX_NONPAE_PFN) {
> >                 max_pfn = MAX_NONPAE_PFN;
> >                 printk(KERN_WARNING MSG_HIGHMEM_TRIMMED);
> >         }
> 
> and it's logically doing the same thing: truncating memory at
> MAX_NONPAE_PFN.
> 
> How about we reuse 'MAX_NONPAE_PFN' like this:
> 
> 	if (IS_ENABLED(CONFIG_X86_32))
> 		memblock_remove(PFN_PHYS(MAX_NONPAE_PFN), -1);
> 
> Would that make the connection more obvious?

Yes, that's better. Here's the updated patch: